Have you noticed a lot of people turning 90 lately?

OK, maybe not a lot. But President Biden, 80, is saying he plans to run for re-election in 2024. His fans are going to be super-aware of anything that suggests he isn’t all that old.

I was thinking about this when I got invited to a 90th-birthday party recently — shortly after I went to a lunch a friend threw to celebrate her 95th. Kinda wondered if I was looking at a new trend.

Yeah, the really-truly-older cadre is zooming — thanks, boomers! The U.S. Census Bureau estimates that by 2060, the number of people 85-plus will have tripled compared with 2017 and the country will have half a million centenarians.

Today we’re not going to discuss the social-support angle; obviously, many of these folks will need a lot of care. Or the fact that while the old are getting older, overall life expectancy in America has actually been dropping, thanks to guns, drugs and Covid.

Now we’re just going to focus on what all these post-90s mean to the way we view the world. Is Biden being 80-plus a big deal when Gloria Steinem just had a party to celebrate her 89th?

“Ninety is the new 60,” declared Muriel Fox, one of the founders of the modern women’s movement. “I’ve got several great friends in their 100s.” Fox recently invited some pals to her turning-95 luncheon, and I’m really liking the idea that being a feminist pioneer can add decades to your life span.

Steinem, who’s off celebrating 89 with a tour of Zambia, is feeling mellow, too. “I’m very conscious that I’m already past the average life expectancy,” she told me via email. “Yet also past many worries about job, family, saving for the future — this is the future!”

Of course, not everybody’s so enthusiastic about hitting markers that make our president look youthful. “It’s something I don’t think about,” said the composer John Kander, who, at 96, is currently busy publicizing “New York, New York,” his new Broadway musical.

The people who really need to get their heads around the age thing are younger. It’s clearly dumb to treat the … older as a doddering population of great-great-grandparents. People will just start wondering whether you’re the one who’s out of it.

For instance, the other day the Republican speaker of the House, Kevin McCarthy, pressed his demands for a negotiation with Biden on the budget by sniping, “I would bring lunch to the White House. I would make it soft food, if that’s what he wants.” Now, McCarthy is a mere 58, but this kind of thing is just going to encourage people to compare his pathetic performance as speaker with that of his predecessor Nancy Pelosi, who held the post until this January, when she was 82.

None of this is to argue that Biden’s plan to run for re-election (announcement coming — soonish!) shouldn’t lead to serious discussion about age. He’d be 86 when a second term ended. Hard to think of a whole lot of 86-year-old world leaders who are currently on the job. Well, Pope Francis. King Harald V of Norway. And if we’re saluting the 90-somethings, we have to add President Paul Biya of Cameroon.

Biden was already the oldest president ever elected when he came into office at 78. Second oldest was, um, Donald Trump at 70.

I would love to cite a list of really great presidents who served in their 70s, but alas, the third oldest at the time of his election was Ronald Reagan, 69, who was eventually rumored to have Alzheimer’s, followed by William Henry Harrison, 68, who died a month into his term.

Dwight Eisenhower, who served from age 62 to 70, starting in the 1950s, was one of the oldest presidents in office. That was back “when 62 was considered really ancient,” said the presidential historian Michael Beschloss.

And Ike seemed older — while he was in the White House, he suffered from a heart attack and a stroke. “It’s never been age; it’s always been illness,” Beschloss said.

Yeah, the number doesn’t matter nearly as much as the condition. And how you appear to the outside world. My friend who had the recent 90th-birthday party doesn’t want to be quoted by name because he’s doing a lot of very high-end, action-packed projects and wants the folks he’s working with to judge him by what he does, not the calendar.

But said friend is an expert on making things look good on camera, and he complained that Biden is being “very, very badly served” by handlers who have him slowly walking onstage when he’s about to give an address. The president, he noted, doesn’t exactly “spring to the podium. Why don’t they just have him sitting at a desk like — hello — F.D.R.?”

Still, Biden on a bike looks a lot better than Trump on a golf course. We will not discuss or even contemplate Trump on a bicycle.

With the two of them competing to be president in their 80s, there does seem to be a real opening for somebody who’s ambitious and younger. Like, say, Gov. Ron DeSantis of Florida, who is 44.

But the absolute opposite is happening. DeSantis is giving youth a bad name.