Ronald Reagan's operatives are taking the initiative on the age issue by organizing a big celebration for Reagan's 69th birthday.

George Bush is running three miles a day and celebrating his own "vitality," a quality that campaign manager Hugh Gregg predicts will parade him right into the White House.

And longshot candidate John B. Anderson is mocking the focus on Reagan and Bush by joking that he will win all the New Hampshire delegates to the Republican National Convention.

It's all part of the quadrennial, media-centered competition known as the New Hampshire primary, a contest which in the past has launched some national political careers, ruined others and sent presidents into early retirement.

In a state where conventional political wisdom often seems as useful as a leftover Muskie for President button, there already have been two swings of sentiment on the Democratic side, and a third may be developing.

In the GOP primary, the conventional wisdom continues to be that Reagan has a solid lead over former ambassador Bush, Illinois Rep. Anderson, former Texas governor John B. Connally, Tennessee Sen. Howard H. Baker Jr., Illinois Rep. Philip M. Crane and Kansas Sen. Bob Dole.

But there are some unconcentional stirrings in a primary many GOP politicans believe could change as quickly as New Hampshire's present a typical sunny and snowless winter.

Bush, after a year of solid work, is drawing big crowds almost everywhere. Anderson and Crane, perhaps helped by their performances in last week's Iowa debate, are attracting support on the left and right wings of the Republican Party. Baker, after what he confesses was a "horrible start" he attributes to his defeat by Bush in a Maine straw vote, is beginning to show signs of organization.

Among the Democrats, Sen. Edward M. Kennedy of Massachusetts was regarded as almost a sure winner over President Carter and California Gov. Edmund G. (Jerry) Brown Jr. when Kennedy announced his candidacy two months ago.

Then, the national rallying to the president after the seizure of American hortages in Iran and Kennedy's stumbles along the campaign trail dampened volunteer enthusiasm for Kennedy and gave a boost to Carter's organizational efforts. Some Carterites even began to hope for a surprise victory in New Hampshire that would nip Kennedy's candidacy in the bud.

Now, the pendulum appears to be swinging again, as time passes without the freeing of the hostages. The telephones are ringsing at Kennedy headquarters, volunteers are arriving in droves, and a sign chalked on the blackboard reads "Only 47 days to victory."

"There's a very cautious optimism growing," says Kennedy New Hampshire coordinator Dennis Kahin, very cautiously.

Carter campaign aides here are worried about the situation. They have told the White House that the president, who withdrew from a scheduled debate in Iowa, risks political embarrassment in New Hampshire unless he campaigns here.

On Friday, Carter aide Ellis Woodward said there is a possiblility that the president might accept an invitation for a Feb. 19 debate with Kennedy and Brown, a week before the New Hampshire primary.

Such a debate could revive the lagging fortunes of Brown, a skilled television performer who has no visible organization in New Hampshire. Both Kanin and Carter regional chairman Chris Brown agreed that whatever vote Brown gets is likely to come at the expense of Kennedy.

The early campaigning in New Hampshire has lost some of its luster because of the press focus given the Iowa caucuses on Jan. 21, which Baker calls "the functional equivalent of a primary."

But most politicians here expect that New Hampshire will receive its usual attention in the 35 days between the caucuses and the primary. A majority of campaign officials in New Hampshire say that frontrunners Reagan and Carter would be damaged here by upset losses in Iowa.

This is how the early campaigns appear to be developing for the Republican presidential candidates:

Reagan, a narrow loser to Gerald R. Ford in the 1976 primary, emerged with his organization virtually intact, despite the loss of former governor Gregg to the Bush campaign.

"It's amazing and a credit to Reagan that most of his organization is still with him," says E. Allen (Ace) Parker, a veteran New Hampshire organizer who is a consultant to the Connally campaign.

Reagan's New Hampshire chairman this time is Jerry Carmen, an experienced politician and former state GOP chairman. Carmen says that Reagan is not taking victory for granted and will make seven or eight appearances in the state during February.

The most unusual campaign item so far is a proposed fund-raising birthday party for Reagan, who will turn 69 on Feb. 6. The "Happy Birthday, Ronnie" celebration now being organized by two Manchester housewives came as an idea from national headquarters as an apparent effort to anticipate rivals' gibes about Reagan's age.

Declaring that stamina rather than age is the main issue, the 61-year-old Gregg says that Reagan would be "a 9-to-5 president" unable to perform the workload required of a chief executive. Gregg underscores his contention by distributing campaign cards featuring a picture of a jogging, sweating Bush and the slogan, "Join a Front Runner!" Carmen retorts that "Reagan will be in the White House in 1980 and Bush will be in the Olympics."

Bush says he will continue to decline Secret Service protection in his New Hampshire campaigning. Gregg says this allows him to continue with friendly, folksy campaigning, in contrast to the more sedate Reagan and his large entourage of reporters and Secret Service agents.

Anderson, the dark-horse candidate, has little organization and less money, but he does have enthusiastic support from the state's durable liberal GOP minority. Campaign director Elizabeth Hager predicts that Anderson will win at least 10 percent of the New Hampshire GOP vote and the two delegates that go with this figure.

There is precedent for this forecast. New Hampshire's generally conservative GOP has always had a dissident liberal wing, which produced 20 percent of the party vote for California Rep. Paul N. (Pete) McCloskey when he challenged Richard M. Nixon for the presidency in 1972. If Anderson does even half as well, his candidacy could have the effect of damaging Bush.

On the other end of the ideological equation, Crane is considered likely to hurt Reagan. The conservative Crane was the target of personal attacks by William Loeb's rabidly pro-Reagan Manchester Union Leader newspaper early in the campaign.

But the Union Leader, the only newspaper circulated statewide besides Massachusetts' Boston Globe, is now concentrating its fire on the rising candidacy of Bush, whom it describes as the candidate of liberals, elitists, eastern newspapers and "clean fingernails" Republicans.

Crane's state coordinator, Eugene Shannon, says that his candidate's campaign was revived by the Iowa debate. He predicts that Crane will make a good showing in New Hampshire.

Connally's campaign apparently was damaged by the refusal of Boston television stations to sell him 30-minute and five-minute prime time spots. The former treasury secretary is the premium fund-raiser among all candidates, and the only one to refuse federal matching funds.

While continuing to negotiate for additional broadcast time, the Connally campaign will rely on heavy mailings and telephone vote solicitations.

Baker's prospects are considered the most uncertain of those of any Republican candidate. He is regarded as personally respected by GOP voters, but his organizational efforts so far lag far behind those of Bush and Reagan.

Baker's chief reliance is on mailings to a list of GOP names considered the most complete in New Hampshire and on a television commercial widely used in Iowa which is intended to demonstrate Baker's toughness on Iran.

Dole's campaign, trying to recover from the wholesale defection of his New Hampshire staff a month ago, is almost as invisible as Jerry Brown's. Campaign director David Sands, attempting to put together an organization, describes the Dole campaign as "just one struggle after another."

Unlike past years, the New Hampshire primary this time will not be plagued with long lists of delegates. Only the candidates' names will appear on the ballot, and they will name their delegates afterward from a list submitted beforehand.

In the GOP primary, a candidate must win 10 percent of the vote to gain any delegates. He would get two delegates if he reached this figure. For delegate apportinment purposes the votes of candidates who fail to reach 10 percent will be allocated to the winner, which could mean that a candidate with less than half the vote could get more than half of New Hampshire's 22 GOP delegates.

New Hampshire's population is estimated between 850,000 and 900,000. Early predictions call for a turnout of 110,000 to 120,000 voters in the GOP primary and 90,000 to 100,000 voters among the Democrats.