Just because a man gets to be 20 doesn't mean his adolescence is over. Ask the Democratic Party, which has just been knocked for a loop by two middle-aged males who are acting out teen-age fantasies.

Gary Hart, former U.S. senator, born-again presidential candidate, age 50, and Nicaraguan Defense Minister Humberto Ortega, whose age by the calendar is 40, are making fools of the donkey because they have to have their own way.

Take Hart. He quit the presidential race May 8. He was the front-runner, had the most staff, the highest name-recognition. He got out, he said, in an embarrassingly self-conscious speech -- "I'm happy with who I am . . . . I've become some kind of a rare bird" -- because the news media would pay no attention to the issues.

Now he's back, because, he says, although not in so many words, the other candidates who tried to take his place in the seven months of his absence cannot cut the mustard.

Only he, he suggests, has the beef. That may be, but the beefcake is his problem. Now he's back telling us that he cannot deprive the voters of his ideas about economic reconstruction and "enlightened engagement." They were all ears before but were distracted by accounts of his dalliance with Donna Rice, a model photographed sitting on his lap during a cruise of the good ship Monkey Business.

His much-reported Washington weekend with Rice spoke of recklessness and frivolity, two qualities not much in demand for the leader of the western world. Even voters who think a man has a right to relaxation ask for a measure of discretion in their public officials.

Hart's reentry on the steps of the state capitol in Concord, N.H., site of his first 1984 triumph, has pushed the Democratic presidential pack back into the mire of farce. Little men, he is saying, make way for a giant.

Hart is brainy, no question. He has been on occasion a courageous politician. But to hear him hinting that the nobility of his public service eliminates the danger of his being the butt of low-comedy japes is to wonder how much he has matured since he got out the first time.

He looked grim and gleeful, sheepish and defiant, as he spoke. He knows he totaled the family car, but he believes he deserves another chance. The important thing, he told Ted Koppel on ABC's "Nightline" Tuesday, is that he is doing what he wants to do. He will find himself, like any self-destructive teen-ager, even if he has to lose his party the election.

Teen-age-advice columnist Beth Winship says Hart seems to be looking back at his adolescence, wishing he "could be like that again."

The Democrats' other torment, Humberto Ortega, is the brother of Nicaragua's president, Daniel Ortega, who has shown signs of growing up lately. Humberto at least had the excuse that he was about to be scooped on his own folly by a Nicaraguan defector, when he unveiled his plans for a 600,000-man military force and his hopes of acquiring a squadron of Soviet MiGs.

The Democrats are accustomed to having their peace plans blown up by the Ortega brothers, who unfailingly drop a bomb at a critical moment.

But Ortega's timing beats anything yet. Costa Rican President Oscar Arias was in Oslo picking up the Nobel peace prize for the plan that bears his name. The Sandinistas were presumably negotiating with the contras.

Worse, the Senate was voting yet again on yet another scheme to slip the contras $9 million in aid, with an open-ended amount for "transportation."

The State Department had been holding Sandinista defector Roger Miranda since October for just such a moment. Now he is being towed around Capitol Hill by the State Department's leading contra fan, Elliott Abrams, who knows that the mention of "MiGs" makes hawks of the most resolute congressional doves. The plan, Daniel Ortega later said, was merely a contingency for a beefed-up militia if the peace plans fail. The Senate voted for the aid.

It's too bad the president didn't find out about the MiGs at the summit. All he says is that Mikhail Gorbachev offered to cancel all military aid to the Sandinistas if we did the same to the contras. More revisionism is in progress: If Gorbachev wants to get involved with the kids in charge in Managua, he's not as sharp as we thought.

The Democrat most scalded by the Ortegas is House Speaker Jim Wright (D-Tex.), who put his political career on the line to push the peace. He understood, he said, that Humberto was showing off for the Nicaraguan right wing, which wants the peace to fail as much as the right wing here.

Still, he called Ortega's delusion "ridiculous, intemperate and irrational."

The same could be said of Gary Hart's -- but won't be, because politicians don't say such things of each other.