Is there a tonic to make the 80-year-old feel 15 again?

Who dies of quaint old age in this live-forever century? Those who half a century ago looked at modern medicine and made it dance to the tune of actuarial tables, modestly forecasting an “aging population,” had no idea of the understatement they let float.

Once, seventy-somethings vanished from radar until their demise, debited to old age — if heart attacks and cancer didn’t claim them earlier. Men disappeared before women, at least in America, a country that all but equated old age with retirement-unto-death. In Richard Ford’s 1986 novel “The Sportswriter,” very much populated by men in their late thirties, the consensus among them is that they have three “good” decades left. Clocking out on the eve of decade seven was still entirely okay.

Good-bye to all that.

Now, age seismographers are on the collective brink of a nervous breakdown.

Who dies anymore? Centenarians, maybe, but not without a fight.

Beat poet Lawrence Ferlinghetti nearly made it to 102, a big step past old-school circumscriptions of “old age” for the author of “I Am Waiting.” Henry Kissinger, he who once brokered Cold War détente, hit 100 before checking out, his final incarnation spewing Cassandra-like warnings regarding the state of the world. Likewise, 95-year-old lefty doomsayer Noam Chomsky is still very much at it. One-term president and Nobel Peace activist Jimmy Carter is pushing 100 (and to think that in 1980 many questioned whether Ronald Reagan, then 68, was too old to hold office, and perish the thought of his running for a second term, if that prospect presented itself, at 72. Horrors!).

So, what, then, will constitute old age? No doubt a mood. An inner human sense that enough life has been lived, weariness setting in.

Actors Clint Eastwood and Robert Duvall are well into their 90s — and dozens of others of their vocation are also in their age bracket, making Tom Cruise seem like a teen heartthrob, though now in his 60s. Even rock stars once confidently assigned early deaths, what with drugs and sundry temptations, are in the middle of eighty-something tours. Nobel Prize singer Bob Dylan has landed on a list that includes 90-year-old blues man John Mayall, he who discovered now 79-year-old Eric Clapton. Paul McCartney’s sweet sixties “When I’m 64” would today demand an 84, Paul himself about two years short of that marker.

Friends in New York and France, well into their ninth decades, proudly tell me they feel just fine, thanks so much, and have no intention of calling it a life before, say, 110.

And such a number will soon be the norm for those who, unlike me, have dodged early assaults from life-threatening ailments, some of which may in fact be arrested before the century is out.

And if that happens, the zombie planet will be for real.

So, what, then, will constitute old age? No doubt a mood. An inner human sense that enough life has been lived, weariness setting in. Not a desire for death, mind you, but a nagging internal feeling of déjà vu, and not of the dreamy kind. It’s what happens to the soul when it sees humanity go ’round one too many times with too many errors repeated, whether wars or natural disasters. (“I am so profoundly world-weary,” said my half-brother with a resigned sigh a handful of months before his death at 89.)

And what of plagues, the kind that come like a celestial brush to usher out large contingents of the old-agers? Call it natural genocide, a phrase most on the planet would take offense to, especially ancient presidents and their nearly as-ancient challengers. Didn’t the Holocaust tell us that life matters above all else? Didn’t it whisper the same to the Russian president or the Israeli prime minister? Apparently not, or not in the right doses.

The other day I visited an oncologist who, when he heard my age, 70, all but bellowed, in English, “seventy years young!”

I said not a word but did take out and dust off my mental abacus, whose clicking sounds were very much in the spirit of John Donne’s bells.

Seventy years young, I thought, and asked not for whom the beads clicked.