To kick off his campaign for the Maryland State Senate, Charles A. Docter held a birthday party last week.

The press release that announced it began: "Some folks may balk at telling their age - but not Charley Docter." He was the host for a "public celebration" of his 47th birthday.

Although he insisted that he is not making an issue out of his opponent's age, many voters have perceived Docter's pitch that way."He doesn't have to say 'age'. He just has to say "effective leadership" said one party worker.

Docter's Democratic primary opponent in Montgomery County's 18th District is incumbent State Sen. Margaret Schweinhaut, who has represented the area in Annapolis for 23 years.

She is a woman who hestiates to discuss her age, saying, only that she is "over 70." Some published biographies put her age at 73.

"Age won't be an issue of me," said Schweinhaut, the senior member of the county's delegation in Annapolis. "But I heard my opponent is suggesting that I'm too old to do the kind of job I ought to do."

Docter, who has been a legislator in the House of Delegates for 11 years, is ready to move on to the state Senate, but Schweinhaut, who has been a state senator from the 18th District so long that the office is called "Peg's seat," stands in his way.

In the same vein, the challengers seeking to oust incumbent Democrats from the House of Delegates in the Sept. 12 Primary, and the Republicans waiting in the wings for the November election are saying "It's time for a change" in the heavily Democratic district.

It is primary contest on one is willing to call because of the enormous diversity of District 18. It is possible for a few thousand voters affiliated with any of several active interest groups - senior citizens, Jews, Roman Catholics, tenants, anti-abortionists, business and blacks - could provide the margin of victory for any candidate.

For example, two thirds of the district's dwelling units are apartments and condominiums, and a fifth of its residents are over 60.

The most prominent landmark in the political georgraphy of the district is Rock Creek Park, which slices north-south through the district.

To the west of the park lies stately Chevy Chase, the northward extension of the silk-stocking neighborhoods of Washington. There are more Republicans here than anywhere else in the district although they are still outnumbered 2 to 1 by Democrats.

To the east of the park is Silver Spring, with its business district and Metro station, surrounded by high-rise and garden apartments, many of whose tenants are elderly or members of racial minorities. From there the district unfolds northward into the old comfortable neighborhoods of Woodside, Woodside Park and Forest Glen, up through middle-income regions to Wheaton plaza.

In this region live the bulk of the Democratic voters - outregistering Republicans 3 to 1 in some precincts. Residents of these east-of-the-park communities traditionally complain that they are politically neglected. But in this election year, residents on both sides of the park are finding they have more and more in common - particularly the economic issues of inflation and high taxes. Both Docter and Schweinhaut were careful to include at least one candidate from each side of Rock Creek on the slates they are heading.

"There was a time 10 or 12 years ago when the Chevy Chase residents said they wanted such things as French in the schools and were willing to pay for it, while the people east of the park said they wanted good schools but couldn't pay for that," said Grace Orlandsky, who heads the District 18 Democratic Caucus.

"But the problems of inflation have meshed these two sections," she added. "No one now will campaign on I promise you more.' Rather, they'll talk about efficiency in government and zero-based budgeting."

Because of the agreement on these overriding election-year concerns, the District 18 candidates have sought out other issues to set themselves apart from each other.

Docter, the popular maverick legislato rand consumer advocate who says he is "ready for a promotion" after 12 years in the House, is empahsizing their "clear differences" in his challenge to Schweinhaut. He says he is more "protenant," citing his sponsorship of rent control legislation and bills to set eviction standards. He also voted for Medicaid funding of abortions. Scheinhaut did not.

She, on the other hand, is stressing "my experience, of course, and the fact that I have arrived at a leadership position in the Senate which is helpful to my constituents."

One of three women senators, she is chairman of the Executive Nominations Committee and is widely regarded as a champion of senior citizens' cause.

Schweinhaut says she wants another four years to finish her work for "people in their retirement years."

Docter's ticket includes House candidates Charles F. Kirkley, pastor of St. Paul's United Methodist Church in Kensington, and Myrna Rosen, a school teacher, Kirkley, who calls himself "Charlie the Citizen," said the delegation needs a "fresh face from the grass roots." Rosen, of Silver Spring, is a condominium owner who believes the east side of the park needs "presentation."

Schweinhaut's House slate is filled by Donald Robertson, a lawyer from Chevy Chase whose style as delegation chairman since 1972 has earned him recognition as the hardest-working delegation member: David Scull, a public interest lawyer and government "reformer," and Patti Sher, wife of former County Council president William Sher and a longtime party worker, who lives east of the park.

Also seeking one of three Democratic House nominations is Stephen Hotsko, who lost in a similar primary bid in 1974.

Republican Donald Dalton, a lawyer and party activist, said he will hold the winner of the Democratic senatorial primary "accountable for the corruption" in Maryland's government.

Five Republicans are seeking the three House nominations in the paimary. One of them, physician Daniel Boyles, former president of Maryland Right to Life, and now legislative coordinator for Maryland Right to Life Action, believes that the legislature meets too long and pays itself too much.

Another former Democrat, Lavell Merritt's a black who owns his own small business and has directed social action programs for the Urban League and pushed for development of more minority businesses in Maryland.

Another Republican, Stephen Hodgkins, 22, is a University of Maryland student who is running because people in the legislature aren't exactly people I look up to, people like Marvin Mandel and that crew from Baltimore."

John Dean, a Chevy Chase lawyer who is no kin to the Watergate figure with the same name, is building his campaign for the House of Delegates on a theme of "government encroachment on people's private lives" and the "stagnation of business because of governmental policies."

Republican legislative candidate Lorenza Simmons is vice president of a commerical interior design and construction firm she operates with her husband.