The English have their nannies and the French hire au pairs. But throughout the suburbs that blanket America, it's traditionally been the kid next door who's stepped in for a few hours while parents stepped out.

An archetypal American intitution, the teen-age baby sitter has long been portrayed within a stage set of the 1950s: energetic, bobbysoxer Gidgets who tie up the telephone all night and empty the fridge with huge, indiscriminate appetites.

Today, parents in Montgomery County voice the perennial complaints about teen-age baby sitters who leave dirty dishes piled in the sink or eat all the bananas intended for breakfast. More soberly, they repeat with frustration the longtime truth about baby sitters: it's hard to find one when you need one.

The 1980 Census showed that Montgomery County had 100,346 residents aged 12 and younger, down by about 8,000 children in that age group in 1960. In that same 20-year period, the number of county residents aged 13 to 19 nearly doubled.

Yet despite such an apparently healthy proportion of prospective teen-age sitters to young children, a number of parents say it's still difficult to find teens who are willing to sit, or who can find the time for it in their modern, fast-paced lives.

"A fair number of teen-agers in the neighborhood are interested in sitting, but they're not always available," said Barbara Moskowitz, a resident of Bethesda's Oakmont neighborhood and the mother of two young girls.

"Kids today seem to be so much more socially active," she said. "They're gone constantly . . . . I've had sitters who were out of commission for months" while they were involved in school activities.

Many sitter-seeking parents note that the supply of experienced baby sitters in the neighborhood dwindles noticeably once teens become old enough to hold part-time jobs at fast-food restaurants and retail stores. Teen-agers who may have been happy to baby-sit for the going rate (generally ranging from $1.50 to $2.25 an hour in Montgomery County) are lured to jobs that pay at least the $3.35-an-hour minimum wage.

The most recent census figures indicate that 36 percent of all students in the nation aged 16 to 19 hold jobs, up by more than 3 percent in just five years.

"I guess they think they can make better money that way, but sometimes I wonder if that's really true, after you take out for taxes and things," said Jane Sweet, a Bethesda mother of two pre-school children. "Maybe it helps to have that kind of job experience when you apply for college."

Moskowitz and Sweet talk about the problems of finding a good baby sitter with a determination some people reserve for running marathons.

Yet, despite even the most aggressive advance work to find a good baby sitter, "a lot of it is luck," said Sweet, echoing what most parents have long known.

Word of one family's success in finding a reliable sitter has always spread like fire through a neighborhood, and the reputation of someone like Vince Manganiello keeps his phone ringing.

"My calendar is usually filled for Saturday nights several months in advance," said the Walter Johnson High School freshman, who will be 15 this month. "Last month I had jobs all five Saturday nights, and some months I baby-sit several Friday nights and a couple of Sunday nights also. I'd say I have to turn down a weekend job or two at least once a week."

Manganiello said he began baby-sitting a year ago "sort of by accident," when a friend called him to say she knew a family who needed a sitter. Since then he's learned a lot about supply-and-demand.

Earlier this fall he upped his rate to $1.75 an hour, and he recently informed the several families he sits for regularly that his rate will escalate to $3 an hour after 1 a.m., "mainly to see if it works, getting them home by 1 a.m., I mean.

He prides himself on having a good rapport with the children he sits for regularly: "It's got to be my favorite part, when I come to the door and the kids scream, 'Vince is here!' and they're really glad to see me."

Manganiello also shares an after-school baby-sitting job with his 16-year-old sister; he says they're saving to buy a used car together. Last summer he also was hired by a group of mothers to work three morning a week with their preschool children in a back-yard "camp." Six mothers each paid him $10 a week, and the money he earned was a hefty chunk of the $500 to $800 he estimates he's saved from baby-sitting in the past year.

While Manganiello has been able to expand his baby-sitting services into a lucrative part-time business, there are some teens with regular part-time jobs who still make baby-sitting as much a part of their teen-age life styles as football games and proms.

Cindy Worrell, 17, of Rockville holds two part-time jobs for a total of 20 hours a week; she spends Saturdays working at a local shoe store and works five days a week after school at the Children's Learning Center in Rockville. Still, the Peary High School honor student baby-sits "four or five times a month," usually for $2 an hour.

"I just love kids," said Worrell, who has been baby-sitting for two years. "I wish I was little again myself."

Worrell, who is the youngest of four children in her family, said baby-sitting came naturally to her because her family has "sort of a tradition of the older kids looking after the younger ones." Yet not too many of her friends baby-sit, she noted, either because "basically they don't have the patience" or because their mothers "don't want them to have the responsibility for someone else's child."

The supply of dedicated teen-age baby sitters like Manganiello and Worrell can't seem to meet the demand, many families agree, so a number of young parents today are trying innovative arrangements such as baby-sitting co-ops and sitter-sharing.

Moskowitz helped organize a baby-sitting co-op with about two dozen other families in her neighborhood. She said her husband, a lawyer for the National Labor Relations Board, occasionally shares co-op duty with her by baby-sitting at night in another co-op member's home.

Moskowitz said the co-op has worked well for other stay-at-home mothers like herself, "because during the day you usually can't find teen-agers available."

At the same time, membership in the group has been dwindling, she said, because "more and mothers around here are going back to work, and when you work full time you don't particularly want to spend your free time baby-sitting."

Some parents in Montgomery County say the baby-sitter gap is making it necessary to hire sitters who are younger than their counterparts of years ago, some who seem hardly more than children themselves.

"Sometimes the situation is pretty bad," said Moskowitz. She recalls an occasion when one mother in the neighborhood left her three young children with a 12-year-old for the weekend. "I was astounded," said Moskowitz, adding that the mother "reassured herself that the girl's parents were nearby."

Recently, Suburban Hospital in Bethesda sponsored a free Saturday morning baby sitters' clinic. The sea of 150 young faces in the group reinforced the general observation of many Montgomery County parents that the pool of available baby sitters appears to be shifting downward from older teens to their younger brothers and sisters.

Those who registered for the clinic ranged from 10 to 16 years old, said Beth Veihmeyer, who organized the clinic. Nearly all indicated they'd already done at least some baby-sitting.

Veihmeyer said she even received calls from a couple of mothers who were interested in signing up their 7- and 8-year-old daughters.

"A lot of the interest (on the part of parents) is undoubtedly due to the phenomenon of 'latch-key' children," said Veihmeyer, referring to those children who let themselves into the house after school and wait alone or with younger brothers and sisters for mom or dad to come home from work.

Veihmeyer said the idea of organizing the baby-sitting clinic grew from a collective concern on the part of the hospital staff about the growing problem of many children in the community being left unsupervised or in the care of young sitters, many without sufficient knowledge of what to do in an emergency.

Dr. Peter Fahrney, director of emergency services at the hospital, spoke to the baby sitters at the clinic about how to handle medical emergencies. He said his interest in the clinic "first came about because of our concern about children who came to us needing treatment but we couldn't locate their parents" to get authorization. Hospital policy requires parental approval before treatment can begin for anyone under 18 years old, Fahrney said.

To help ease that problem, Fahrney's department recently printed an "authorization for treatment" form that parents are encouraged to complete and keep on file at the hospital. He said the idea has been "well-received" in the community, and he encouraged the baby sitters to circulate the forms among families they sit for.

A policeman and a fireman briefed the youngsters on what to do to get help quickly and how to respond when a fire breaks out. A staff psychiatrist at Suburban Hospital outlined child development in elementary terms, to help the baby sitters better understand why the behavior of the small children they look after is often so erratic and bizarre.

Emergency room nurse Jean Oddes emphasized to the group the importance of having a positive attitude toward the children they sit for.

Speakers emphasized the need for sitters under 16 to have a "backup" person, a parent or neighbor close by to consult in case trouble occurs or a child becomes unmanageable.

David Schwab, 10, and his 12-year-old sister Rebecca, of Rockville, attended the clinic. Their father, Paul Schwab, said he and his wife encouraged their children to attend the clinic because "we've always tried to be fairly safety-conscious with them."

He said his son and daughter, as well as other kids their age, occasionally baby-sit in their Walnut Woods neighborhood, "in very sheltered situations."

"Typically, they baby-sit for short periods during the day, or in the evening when kids are only up until 7:30 or so," Schwab said. "Usually it's for kids about 3 to 7 years old, not the kind of situations where they would have to spend a lot of time changing diapers" and looking after infants.

David Schwab said he and his friends usually earn from 75 cents to $1.50 an hour.

Diane Mrsny, a single parent, brought her 11-year-old daughter Jill to the baby-sitter clinic. She said her daughter sits occasionally during the day, "and I wanted to help bring her along."

"Kids today are more responsible," Mrsny said, noting that more mothers today appear to be seeking baby sitters in her daughter's age group.

"You seem to have to get them younger these days," said Barbara Moscowitz. She said she prefers to use baby sitters who are at least 14, or maybe 13 under special circumstances. But obviously, she stresses, the question of age is only one criterion parents must consider in hiring a sitter.

"I've had some older sitters I wouldn't leave my kids with again." she said. "There are even 17-year-olds who are incredibly scatterbrained. I also have some sitters my kids don't think are as much fun to have around, but who are more responsible."

Moskowitz, who did some baby-sitting herself as a teen, said baby sitters today "seem to better prepared for emergencies than when I when I baby-sat as a kid." She thinks that may be because parents are demanding it: "Parents of my generation are so paranoid. We're always talking about 'these times, in this environment.' "

Whatever the motivation, parents in Montgomery County seem to agree the baby-sitting clinic is a good idea to help local baby sitters take their child-care responsibilities seriously.

Often, a baby sitter is "clearly just a kid," Moskowitz said. "You tend to forget that sometimes."