William Henry Harrison, at 68 the second oldest man to become president, caught a cold on Inauguration Day and died 30 days later of pneumonia in 1841. His vice president, John Tyler, whom Theodore Roosevelt later described as "a politician of monumental littleness" became president.

This fact and the "age issue" it symbolizes haunted Ronald Reagan, who turns 70 next month, throughout much of his long quest for the presidency. Now is a central fact of life for Vice President-elect George Herbert Walker Bush.

Quite simply, Reagan's age will make Bush's vice presidency different. It will shape the way the new vice president acts and what he does.

The presidency ages men cruelly, and many potential presidential candidates think it unlikely that Reagan would run for reelection in 1984 at age 73.

No matter how much he protests to the contrary, Bush will find it hard to ignore the prospect that he could rise to the top job in the White House in a few years -- or blow it all.

This should reinforce Bush's natural instincts. He is my nature a cautious man, a team player, whom his critics say gets along by going along. Conflict makes him edgy and ill at ease. He does not make waves; he makes friends. He is a compromiser, not an ideologue.

Bush, 56, is the quintessential government insider. He has proven himself a loyal and effective servant to two previous Republican presidents, Richard Nixon and Gerald Ford, in a host of positions and should have no trouble doing the same for Reagan.

As a running mate, Bush was self effacing almost to a fault. Not once did he try to upstage Reagan. As vice president, he will be unobtrusive. "I will keep a low profile, and I will not have hurt feelings when people ask, 'What happened to George Bush?'" he said in one interview.

Bush would like to model his vice presidency after that of Walter Mondale, as a trusted adviser and confidant of the president. He has been assigned Mondale's office in the West Wing of the White House and been assured he will be able to attend the same meetings that Mondale did and have a weekly luncheon with the president.

Bush's top assistant, retired admiral Frank Murphy, also will be included in senior White House staff meetings, according to sources.

But Bush realizes his job will be pretty much what Reagan says it will be, and so far Reagan has said very little.

Bush believes that whether he emerges as a major force or a bit player in the new administration depends on how well he succeeds in winning Reagan's trust and good will. "If I don't have his confidence," he quipped at one point in the fall campaign, "I'll be going to funerals in Ecuador."

The transition has been a ticklish time for Bush. Reagan has often appeared aloof and distant from selection of his Cabinet and formulation of policy. Bush has taken care not to overshadow him. He has been sent to one funeral, that of former House Speaker John McCormack, and has accompanied Reagan on his three forays to Washington.

Bush has spent most of his time putting together a 25-member staff and cementing relationships with Reagan's inner circle. His longtime friend and former campaign manager, James A. Baker III, has become Reagan's chief of staff. Yale buddy and Connecticut campaign chairman Malcolm Baldrige has become Secretary of Commerce.

Few men have ever come to the vice presidency with more experience in government than Bush, a former congressman, ambassador to the United Nations, envoy to China and Central Intelligence Agency director. And few men have come to it with fewer commitments on what they would do in office.

Reagan and Bush barely knew one another until the Republican National Convention last July. Not only were they near strangers, they also were longtime rivals for the Republican nomination. Reagan had misgivings about Bush's character and ability to function in crisis, and Reagan's conservative supporters suspected Bush of being too liberal.

Like most presidential tickets, theirs was a marriage of political convenience. Just as Jimmy Carter selected Mondale because he was a bridge to liberal Democrats, Reagan picked Bush as a link to GOP moderates.

It is difficult to imagine two men from more different backgrounds. Reagan is a product of mid-America and Hollywood, son of a small-town shoe salesman. Bush is a product of a far different class, the moneyed and established Eastern elite. His father, Prescott, was managing director of the banking house of Brown Brothers, Harriman and then a senator from Connecticut.

Reagan grew up in a series of small, faceless Illinois towns and graduated from tiny Eureka College. Bush grew up in Fairfield County, Conn., in a world of stone mansions, wood paneling and vast lawns. He graduated from Andover and Yale, then redoubts for the WASP elite.

Reagan grew wealthy as an actor and as spokesman for General Electric. Bush grew wealthier as an oilman. He named his company Zapata Petroleum after an old Marlon Brando film, "Viva Zapata!"

Reagan relaxes by riding horses at his ranch; Bush is a Texan without a cowboy hat or a cow. His idea of a good time is a three-mile run, a hard-fought tennis match or an afternoon spent running a speedboat at his family hideaway in Maine.

During the primaries last year when Bush was trying to draw attention to his age difference with Reagan by jogging each day, he said, "I don't think we're running for athlete of the year. But if we were, I'd win."

Once Bush became Reagan's running mate, he went to extraordinary lengths to be a proper supporting player, even when it meant minimizing his support of the Equal Rights Amendment, which Reagan opposes, and his earlier criticism of Reagan economic policy, which he once called "voodoo economics."

The two men maintained frequent contact during the campaign, but Bush was included in few strategy sessions and decision-making sessions. The two are said to have developed a good working relationship and growing mutual respect, perhaps because they have more in common than is obvious on the surface.

Privately, both are instantly likeable men who enjoy a good story and a hardy laugh. Both began their political careers in 1964 as outspoken supporters of Barry Goldwater. Both are optimists about the country.

Yet Reagan has only hinted about what he wants Bush to do. He has said he would like him to concentrate on foreign affairs, national security and congressional relations, areas in which Bush has experience.

By law, Bush's only duty will be to preside over the Senate in session -- a job so minor that it is passed among junior senators when the vice president is absent -- and case a tie-breaking vote in the rare case of deadlock.

Bush has avoided other specific duties on the theory that they would keep him from dealing with broader policy issues. He has said he would like to be a generalist, like Mondale.

"What I want is for people to wake up in two years and say, 'You know, this guy did something,'" Bush said in a recent Time magazine interview.