Washington ? Rumors of impending retirements among the over-70 set on the Supreme Court pop up periodically in the nation’s capital. This is the longest stretch in modern history without a vacancy, and many court-watchers had assumed that one or more of the aging justices would step down before the new session gets under way on the first Monday in October. This spring or summer would have been an ideal time for two of the Republican stalwarts, Chief Justice William Rehnquist, 77, and Justice Sandra Day O’Connor, 72, to announce they were stepping down. With President Bush enjoying record popularity, the GOP would have been on a strong footing to confirm conservative replacements. But if historical trends hold, the president’s party could lose seats in November, which would put Bush in a less commanding position on Capitol Hill.

With the future makeup of the Court so critical to implementing the Republican agenda, why would Rehnquist and O’Connor gamble with the political process and postpone their retirement? The answer is probably no more complicated for them than it is for a growing number of senior citizens who choose to work beyond what we regard as normal retirement age. They don’t feel old; they don’t feel compelled by the number on their last birthday to drop out of the work world, and together they are changing the definition of what we consider old age.

Thanks to medical science, somebody who is 65 doesn’t have to come across as elderly. And as the oldest of the baby-boomers approach 60, they’ll extend middle age to 70 and beyond. With replacement parts to keep hearts beating and knees flexing, today’s 75-year-old, or even 80-year-old, is comparable to yesterday’s 65-year-old. Astronaut John Glenn, the first American to orbit the earth, rekindled the experience decades later, joining the crew of a space shot and proving that at 75 he was still fit. Former President Bush parachuted out of a plane to mark his 75th birthday, demonstrating that extraordinary physical feats aren’t the sole province of the young.

Ronald Reagan won the presidency at age 69, and left office at 78, the oldest president in history. The idea of electing a man in his 70s to a second term as the most powerful leader on earth seems preposterous on its face, but Reagan won in a landslide, despite a debate performance that set off whispers that he might be suffering from Alzheimer’s. Reagan’s strength as a leader overrode qualms about his age. And though he faltered at the end of his time in office, his presidency is rated among the strongest of the 20th century.

Vice President Dick Cheney is a triumph of modern medicine. After suffering four heart attacks, the first when he was in his 30s, and having a defibrillator installed in his chest, he is active in the effort against terrorism and the maneuvering against Saddam Hussein. Not too many years ago, Cheney’s medical history would have disqualified him from such stressful work. Now medical science touts him as a success story.

Extending longevity is a good thing, but it has its limits. South Carolina Sen. Strom Thurmond should have retired years ago, given his intermittent mental acuity, yet his fellow lawmakers, many of them no youngsters themselves, humored him as he prolonged his career beyond his capability to perform.

Prediction: Look for senior citizen discounts, normal retirement ages and even the Social Security benefit age to increase to 75.

Political correspondent Eleanor Clift contributed to this column.