"Middle age," said Ronald Reagan, "is when you're faced with two temptations and you choose the one that will get you home by 9:30."

The former California governor and unsuccessful candidate for the GOP presidential nomination was in Alexandria last night celebrating what he called the 27th anniversary of his 39th birthday. According to the Gregorian calendar, he was 66 years old.

Accompanied by his wife, Nancy, Reagan chose to observe the occasion with former campaign aides, Reagan loyalists, journalists who had dogged his trail to Kansas City, the frankly curious, like Agriculture Secretary Robert Bergland, and former Reagan running-mate Richard Schweiker.

When Reagan spotted Schweiker, he grabbed him for an especially warm welcome that indicated a genuine fondness for the Pennsylvania Republican senator. Nacy Reagan kissed Schweiker and his wife Claire, asking to be remembered to their children.

Schweiker talked to some guests about what he called a new Republican spirit these days in the Senate. "We're convinced if we don't all hang together we'll hang separately," he said.

Someone else talking about spirit was White House energy adviser James Schlesinger, who last spring had advised Reagan on foreign policy and events. Standing in front of the fireplace, into which logs were fed regularly to augment the central heating. Schlesinger talked about a changed of spirit on energy conservation and predicted that by cutting back, Americans can save as much as 30 per cent in energy consumption.

But rather than be disciplined like children or dogs, Schlesinger told public relations executive Robert Gray, Americans are best when self disciplining themselves.

Reagan, in town for a conference of the American Conservative Union, was clearly the favorite of its 550 delegates. But if some people at the conference and at the party last night were thinking about another Reagan try in 1980, others were wondering about his age.

Reagan himself could hardly have looked or sounded more chipper, joking about being 39 and feeling like it.

"God bless Jack Benny," said Reagan, "he spared us all the middle age."

But when someoe asked Reagan if he might try another run four years from now he only laughed. "Let's wait until along about 1980 and we'll see," he said.

Nancy Reynolds, form aide of Reagan's when he was governor of California and more recently press secretary to Mrs. Reagan during the campaign, managed to round up a representative cross-section of Reagan friends and observers that included former campaign director John Sears, former campaign chairman Sen. Paul Laxalt, former press secretary Jim Lake, Secret Service agent Ed Hickey, several senators and representatives, and such journalists as Frank Reynolds, Tom Pettit. Mary McGrory, David Broder and Lou Cannon.

Flying up from Florida especially for the party were Cdr. John McCain, held prisoner by the North Vietnamese for 5 1/2 years, and Cdr. Everett Alvarez, the longest held American POW (nine years in Hanoi.)

Agriculture's Bergland was there becuase of his friendship with Jim Lake. "There's a time," said the former Minnesota farmer, who was meeting Reagan for the first time, "when partisanship is worth a candle. I rather like him."

When the fire on the hearth nearly died out, Schlesinger tried to revive it by placing a log on the grate. It nearly put the flame out, paving the way for Sen. Jesse Helms of North Carolina to come to the rescue with two other logs that immediately caught fire.

Everybody laughed, prompting Rep. Phil Crane, new ACU chairman, to get in a little partisan dig. "At least we're on top of it," he said.

When Schlesinger left, Nancy Reynolds thanked him for coming, then asked how she should address him. "Mr. Secretary or Mr. Ambassador?"

"Just call me 'Mr. Czar'," Schlesinger said, slipping out into the night.