President Reagan is 70 today, a milestone once beyond the life expectancy of most Americans and reached by only one other White House incumbent, Dwight D. Eisenhower.

A year ago, age was an issue that many thought would keep Reagan from winning the presidency. His campaign was reeling from a loss in the Iowa caucuses, and the approach of his 69th birthday was looked upon with misgiving. In youth-conscious America, being elderly was thought to be a possibly crippling political disadvantage.

"The Gipper's too old. He's just too old," people said to explain why Reagan couldn't make it to the White House.

What a difference a year makes. The Gipper, as some call Reagan after the Notre Dame football star he played in his first widely acclaimed movie role, is celebrating his 70th birthday in the White House with a barrage of age jokes.

Although only Eisenhower turned 70 while president -- and he celebrated that birthday shortly before the end of his second term rather than three weeks into his first -- other nations have been led by men in their 70s and even their 80s.

Konrad Adenauer was 87 when he ended his 14 years as West Germany's chancellor. Chairman Mao Tse-tung was 82 when he died. China's present ruler, Deng Xiaoping, is 76, and Taiwan's president, Chiang Ching-kuo, is 70.

Soviet leader Leonid I. Brezhnev is 74, and despite the harsh U.S.-Soviet rhetorical exchanges that have marked the first weeks of the Reagan administration, the two leaders have in common that each was described as middle-aged as he turned 70.

Reagan jokingly labeled himself middle-aged; the Soviet hierarchy was not smiling as it hailed brezhnev. "Seventy years of life is not little, but then it is not much either. It is good that in our country this age is considered only middle age," said Andrei Kirilenko, a Politburo member.

No Soviet marred the Brezhnev celebration by recalling that during a similar glorification of Nikita S. Khrushchev on his 70th birthday, Anastas Mikoyan had said:

"We are celebrating today the jubilee of a middle-aged man who is, as you can see for yourselves, at the peak of his strength and capabilities." Six months later, Khrushchev was ousted on the grounds of his "advancing age and deteriorating health."

Reagan, the oldest man to be elected president of this country, presides over a nation that also has grown older in the last 20 years.

In 1960, 9.2 percent of Americans were 65 or older. Now, 112 percent are in that age category. That is about 25.5 million people who, as politicians have discovered, are far more likely to vote than are other age groups. In 1980, 65 percent of them voted, making them 16.8 percent of all voters.

Reagan, however, moved among elderly Americans during the campaign like a man of another generation. Audiences remarked how young and healthy he looked. At Leisure World in California's Orange County, Reagan appeared with GOP Senate candidate Paul Gann. One gray-haired woman turned to her companion and said: "It's hard to realize that Gann and Reagan are the same age."

Although Reagan campaign staff members feared that the age issue would jump up and harm their candidate sometime, somewhere, it never did, and they relaxed enough to begin affectionately calling their leader "the Oldest and Wisest," a phrase they had feared when it was first used.

Rep. Jack Kemp (R-N.Y.). used the phrase when he introduced Reagan the day after Reagan announced his presidential candidacy, but it wasn't until well after the Republican National Convention that Reagan aides started using it.

Reagan's health has been consistently good. He has had an annual physical exam since 1957, and his Los Angeles physician, John Reynolds, describes him as being in "remarkably good physical condition." The National Center for Health Statistics says he has a statistical probability of living to 80.

Only occasionally are there public signs that Reagan is aging. His hearing has deteriorated, but he does not need a hearing aid. It is well known that during the campaign he napped in the afternoon if he had an evening appearance. He ran leisurely campaign by the hectic standards of this age, usually starting his day late or ending it early.

Reagan handled the feared campaign birthday a year ago by holding an elaborate series of parties on the theory that one might as well celebrate what cannot be concealed.

He joked that being 69 was better than the alternative.

This week he has stolen a page from Jack Benny, and says he is celebrating the 31th anniversary of his 39th birthday.

Not every Reagan is so candid and relaxed about age.

Nancy Reagan's age is in some dispute. The First Lady says she was born on July 6, 1923, which would make her 57. However, two of her cousins say she was born in 1921. Records at her alma mater, Smith College, support the cousins.