For years, Charlie Jester and his wife, Esther, have lived the sort of existence usually associated either with carefree youth or great wealth. They have toured Europe and North Africa, gone skin diving in California, bummed around the country and down to Mexico in a travel trailer. Now he dabbles in politics and is learning to paint.

Jester, 73, is a retired New York City police detective with a $3,400-a-year pension. After he retired the first time, he continued to fatten his nest with a second career -- running a lakeside concession stand north of New York City, selling hotdogs and beach equipment.

As a result, he also gets about $2,300 a year in Social Security, and his wife receives roughly $1,400. Three years ago, after selling their first home, they bought a home for $14,000 here in the pastel warmth of this Gulf Coast retirement mecca and settled down.

The stereotype of retirement often lumps in all the harhest images of old age, from physical to economic frailties. And of course many old people, especially women alone and minorities, are poor and disabled, their fixed incomes ravaged by inflation. But this is only one slice of the picture.

Another group of older Americans, numbering in the millions, forms what amounts to a new leisure class. Figures from the census and other surveys show that the current crop of maturing Americans is reaching the state loosely referred to as retirement in better health -- both physical and financial -- and with more choices of how and where to live than any generation before them.

This is largely a result of improved pension plans and other benefits tailored to their needs. In 1930, specialists point out, there was no Social Security, and pensions of any kind were limited to a few workers.

Moreover, there are indications that a significant proportion of today's retirees supplement their retirement income by starting a second career, as Charlie Jester did, or by mixing part-time or seasonal work with leisure.

Some of this work is done in what specialists call the "underground economy," and does not necessarily show up in in government figures.

Couples like Charlie and Esther Jester are part of this broad, middleclass group of retired citizens who up to now have been only vaguely defined by public policy makers and social researchers.

They are increasingly visible economically and politically, particularly in the trailer parks and residential clusters of retirement communities such as the breezy resort, one of the fastest growing cities in the nation.

Just one-seventh of the nation's over-65 population now lives below the official poverty level, for instance, compared with a full one-quarter who fell below it in 1970, according to the American Association of Retired Persons (AARP). And a congressional report in 1977 indicated that figure might be as low as 6 percent of the elderly, if non-cash federal benefits were included in the figuring.

Only 6 percent of all people over 65 are institutionalized, surveys shows, and most not only live in the community but head their own households rather than living with their children.

There is a growing tendency that is "almost hedonistic" among many in the large middle group, observed Lloyd Wright of AARP. "Before, the parents used to build up an estate to leave to the kids. Now they're saying, well, the kids are doing better than I was at this point. We're earned it. We're gonna enjoy it. . . I think it's a major shift in values, and I don't think it's necessarily a bad shift."

Advertisers are just beginning to discover the "aging Pepsi generation" as consumers. "Smaller houshold size gives olders their edge in purchasing power," bugled a recent marketing newsletter, calling its subscribers' attention to the growth in numbers and spending power of this segment of the population. "The 55 to 75s are where the action is in 'Mature America' . . . enjoy spending money, lavish it on themselves, do it without any guilt."

There are 18.5 million citizens in the 60-69 bracket along, the publication noted. While retirees naturally have less income than when they were employed, specialists say, with children grown and major purchases already made, they may need less.

Their mobility gives them, among other things, a significant influence on the distribution of the American population, with implications for related public policy, according to the Census Bureau.

About 3 million households, or one-fifth of all those who moved from one state to take up residence in another during a recent three-year period, reported some kind of retirement income, according to a study by Census Bureau analysts Larry Long and Kristen Harsen.

But only 3 percent of these interstate migrants reported "retirement" per se as their reason for moving. Rather than economic reason, they are more likely to have listed climate, or nearness to relatives.

This mobility among retirees (and also other groups, such as the growing numbers of people of all ages who live alone) "implies a rise in the degree to which jobs follow people, as opposed to the somewhat more traditional process" in which people chase jobs, the census specialists said.

A majority of all retirees do not move far from home, but still the impact of retirees and semi-retirees who do move goes beyond their numbers alone, the specialists noted, because the retirees' movements create employment for others who are interested primarily in jobs and earnings -- in leisure time or health-care services, for instance.

Along Florida's Gulf Coast, as in much of the state, the variations of old age can be glimpsed in the high concentrations, from the wealth in the high rises out on the Keys to the struggling poor of the inland scrub.

Florida already had the oldest average population in the nation, with about 24 percent over age 65 and 29 percent under 20. And some time during the 1980s, Florida is expected to become the first state where the number of deaths exceeds the number of births.

Younger people in the area, in constant contact wih such an array of elders, find themselves refining their notions about aging. "If changes your whole perspective," said one thirtyish writer in the area. "You don't think about people 55 and 65 or even 75 as old anymore. They are the 'young-old.' You see so many people in their 80s and 90s."

Some retirees have shied away from such conspicuous concentrations of others their age, migrating to places the experts never anticipated, such as the Ozark mountain region of Arkansas, according to the Census Bureau's Long.

After shopping around the country for a place to settle, Charlie and Esther Jester picked Sarasota because of the climate, the various tax and other breaks afforded those past a certain age and, Jester added, "for the culture. You know, this is the cultural center of Florida, for its concerts, art and things like that. It's especially good for senior citizens. They actually taught me to paint here!"

Jester and his wife, who have a married daughter and grandchildren, are active in volunteer work, and he represents his district on the Silverhaired Legislature, an advisory political organization that promotes the political concerns of senior citizens.

It was the second career, the lakeside concession, which really gave them a leg up financially, made it possible to travel and still pay for a house, Jester said. "We live very reasonable, my wife and I. I'm not crying the blues."

For Mary Whited, 63, it is a life of bridge, beach, watching the horse races and playing hostess to a parade of daughters and other visitors from the Washington area, where she worked for 30 years for the government. She retired five years ago from a job as a claims representative at the Social Security Administration in Alexandria, Va.

As a federal retiree, her pension is indexed to the cost of living and rises with it. "If it wasn't, things would certainly be more difficult," she said. "I have other friends not indexed and they're worried about what's going to happen if inflation keeps up."

Sarasota offers a good example of how more active, healthy and perhaps wealthy senior citizens are using their retirement energies to help contemporaries who are less well off.

An unusual agency called the Senior Friendship Centers, a nonprofit volunteer outfit, is the coordinator, bringing together people over 60, in all income brackets and states of health, for a mix of activities from language lessons to dances to candlelight dinners to health services.

Under a new Florida law that applies to retired physicians , Dr. Irwin I. Portner, 72, a specialist in family medicine who moved here from Boston, is still able to use his skills. He heads a team of 29 other physicians, along with a staff of volunteer retired nurses, who regularly donate medical services to the health center here.

I would be too bad to let all that medical training go to waste, said Portner, who was stricken with Legionnaire's Disease several years ago and has since recovered. "Most of us have practiced 35 to 45 years."

Joe McMillan, 73, retired to Sarasota after 30 years as a civilian employe in the Command (formerly Signal Corps) in Fort Monmouth, N.J. He had his priorities in order. He wasn't rich, but he knew what he liked. "I liked the climate here, and I liked to fish. I fish from the bridges."

He and his wife moved here and bought a small house. But he found there was a limit to how long he could sit under the hot sun catching sheepshead and mackerel. He wanted something more to do.

"But I refused to punch in to any more time clocks," he said. "I've turned down serveral paying jobs." Instead, he volunteered his services free to teach at a vocational school, install earpieces in TV sets for a hearing aid society, and set up a fishing rod repair course at a boys club. Now he also repairs small appliances, under the auspices of the Friendship Center, for people who bring them in to an old converted filling station on Main Street.

"That's what life is all about," McMillan said. "Forget money. If you don't count your money, you're in pretty good shape."