Well from the last episode, Kelpians went from Living about 20 years (IE - they killed themselves as they entered their form of puberty) to living over 120 years (as that's about the age of the Kelpian in the holodeck of the ship on the dilithium planet); And that's probably without a lot of working medical technology as that holodeck is in rough shape.
Yeah I agree that it's kind of ridiculous that there would be no humans in Starfleet currently that remember the burn. Hell, The Vulcans and Romulans on N'avar should still have plenty of citizens alive (including scientists) who were alive and working in their fields at the exact time the Burn happened. I think that's one aspect of this new world that the writers have so far pretty much glossed over. Especially with respect to the Vulcans considering how long they lived 930 years ago.
One thing that kind of dawned on me some years ago is that the whole thing with Vulcans living a very long time because of some biological differences, while true, was also due to in part the advanced medical technology of the 23rd and 24th century. Human beings may not live as long as Vulcans, but they sure as heck live a lot longer than today. Before I realized that, I thought Vulcans lived over double what a human does, but really, it's more like a third more.
Another point of relevancy in this is it is also sort of explored in the Expanse. The Martian Ambassador is 120 years years old and looks like he's in his late 50s. A Martian Admiral nearing retirement has served for 60 years (in the real world, it's typically 35 years or at the mandatory retirement age of 65, with 40+ years an exceptional circumstances like a very senior promotion). Avasarala is about 90 years old before the 30 year time jump in the books. But things happen in the Expanse over very long time scales due to the distances involved and "realistic" speed of travel.
Human beings living longer and healthier isn't something Star Trek really ever touched on except in the most general sense. And it really didn't have much reason to until now. The Burn almost forces it though because so many characters talk about it like life cataclysm they lived through that left significant psychic scarring in their lifes - a galactic scale 9/11 or Indian Ocean Earthquake - and not something they read about growing up, akin to say, the American Civil War, or the Dust Bowl. This is important for Vance's character in particular. The motivations of a man who wants to restore something he knew in his youth that was brought low, versus the motivations of a man who want to return something he inherited to an idealized state he only ever heard about, are entirely different. The former only works if Vance is like, 200 years old already.
My personal view? I hope that everyone in the Federation in the show who looks late middle aged or older was born sometime in the 30th century. That makes the Temporal War an early formative experience - kind of like fighting World War II when you're 20 - only for the Burn in the 31st century being the unexpected turn their lives took in the prime of their lives... like 9/11 / War on Terror for the boomers. And now as their lives / careers are sunsetting, Discovery appears and offers them one last chance to return the Federation to what they knew when they were young.
But that said, Discovery, while much improved this season, is an intrinsically sloppy show when it comes to details and character arcs like this, so we're almost certainly dealing with characters who age like 21st century humans do more or less, and we're not supposed to think about it much when Vance goes all teary eyed over an idealistic galactic utopia that effectively ceased to exist 85 years before he was born
