Columbus, Ohio ? He was a hero in two of America’s wars, then a fabled test pilot, a four-term senator, a presidential candidate, finally a party elder. But in the mind’s eye and in history, John Herschel Glenn Jr. is frozen in time.

His boyhood friends, his wartime comrades and his Senate colleagues have grown old, and some have died. His political causes are yesterday’s, their urgency gone, the stuff of the past rather than the right stuff of today. But John Glenn remains what he was when he became a staple of black-and-white television and the color pictures of Life magazine, two media themselves both gone:

An American phenomenon — forever young, forever clad in the silvery Project Mercury space suit he wore when he sat above a smoking Atlas booster, forever the commander of Friendship 7, forever the first American to orbit the Earth.

So today brings an important, sobering, even jarring landmark — for him and to some extent for the country. Today, Glenn, who for a generation represented an American future as shiny as his silver space suit, turns 90 years old.

He was born in a country that, in the years just after World War I, was self-conscious about its strength, nervous about its role, reluctant to confront either. He grew up in an innocent time (the 1920s) in an innocent place (New Concord, Ohio) and volunteered to confront evil (World War II) and aggression (Korea). He caught the excitement of the new technology, and then, as one of the Original Seven astronauts, spawned excitement among millions who turned their eyes skyward to the boundless expanses of space and to the boundless opportunities it seemed to offer.

Today Glenn isn’t so much a shadow of himself — so many men at 90 are — as he is a mirror of himself. The face still as round as the sun, the smile still as broad as his Midwest accent, Glenn still walks briskly, still exudes an infectious optimism from behind gold aviator glasses. Last month he flew his Beechcraft Baron between Washington and Columbus. Next winter his wife, Annie, 91 and recovering from knee replacement surgery, plans to ski. The couple looks forward to driving across the country in the fall.

“I’d rather burn out,” he says in his office in the John Glenn School of Public Affairs at Ohio State University here, “than rust out.”

He’s not likely to do either. Glenn always seemed the personification of old American virtues — hardworking, daring, forward-looking, sensible, unpretentious — even when he was young. For four hours and 56 minutes in 1962, a period of Cold War tension and Kennedy idealism, he circled the Earth, then returned to a world that made of him a hero.

Glenn exuded the upbeat spirit of an era he helped define. From Iowa and New Hampshire to states across the country during his disappointing 1984 presidential campaign, he trumpeted a can-do utilitarianism and a sense that all things were possible. One reason he did not prevail that year is that he still believed, while the rest of the Democratic Party of the time did not.

Glenn read science fiction as a boy, then lived it as a man. He and his father flew in the front of an open-cockpit biplane harnessed by a mere leather strap. He later left Earth’s orbit on Friendship 7 protected from what he feared was sure incineration by the straps of his retrorockets.

He built model airplanes, the old-fashioned kind made from balsa, not plastic, then lived to see the old Mercury spacecraft models by Revell selling as antiques or maybe keepsakes for $85.

He returned to space in 1998 for a nine-day mission on the shuttle Discovery. He was 77. That space mission was 45 times longer than his first one 36 years earlier, and about 1/45th as exciting for the nation. In a wheelchair at Cape Canaveral the morning of his launch was his Korean War wingman, a shrunken residue of a wild specimen whom Annie Glenn remembers as the most profane man she ever met. We remember him as greatest hitter who ever lived, Ted Williams.

But his closest associates came from the Original Seven. “We were bonded for a long time,” Scott Carpenter, 86, the only other survivor from that group, said in a recent telephone interview. “I admire John enormously. This is a Mercury friendship.”

Glenn and his six colleagues were reared in a world without the word “astronauts,” but everything about their selection, and then their training, signaled that the nation was embarking on an epic undertaking that melded teamwork and individuality. World War II was won by as many as 16 million Americans (and that’s not counting the British, Soviet and other combatants). Space would be won by seven Americans, or so the fable said.

Now Glenn is 90, still transfixed by the future but knowing it is a future he will not shape.

“We’re a nation that more than any other country has stressed research,” he said the other day. “We’re accustomed to the new and the unknown. It’s the way we grew up.”

So many of us grew up in a world molded not only by what Glenn did, but also by the way he thought. And by the words Carpenter uttered as his friend’s rocket prepared to lift into the Florida sky:

Godspeed, John Glenn.