High fantasy novels often deal with long-lived people: immortal elves. Fantasy authors have come up with different ways for elves to occupy their time. Off the top of my head:

- master crafts (e.g. build guitars for 100 years)

- reminisce (sink so deep into memory that you lose track of passing days)

- count seasons instead of days or weeks ("I'll see you 4 summers from now" instead of "I'll see you tomorrow")

The tipoff is how at the very start they casually talk about the "Methuselah Treatment" in a tone that suggests that it's such a well-known thing that they don't need to explain the first thing about what it is.

After all, if it turned out there was a reliable treatment that extended human lifespan to 1000 years, it would be the biggest news since...well, ever. Or maybe second to "SINGULARITY EXPLODES IN 'BIG BANG': MATTER, ENERGY NOW EXIST".

Yes. It’s categorized under “fiction,” and gets increasingly ridiculous/funny (tastes vary) as you go along.

Yep, my comment was a PSA of sorts, since it seemed some took it for marketing drivel instead of fiction.