And healthy eating, sleep, exercise, being sociable.

These seem within one’s agency.

You're kidding right? Shatner's had a beer belly for the last 40 years and Patrick Stewart has always been quite thin. It's just luck and genetics.

Shatner had the gut, but did you see him use an elevator to get on Jeff's rocket? He climbed the stairs, 7 flights, roughly 70 feet above ground, at 90+ and clinically obese. Was he gasping when he got to the top? Yes. Did he do it? Yes! Did he keep up with the kids half his age? Yes!! Did he complain? No! Did they use a (danny) crane to hoist him up there? No! Could Patrick have done that? I doubt it after watching his performance in Picard. He looked and acted feeble in that. Patrick was thin, but that doesn't mean fit. Shatner was fat, but strong.

I only watched the first season of Picard (and, sadly, didn't like it much), but I got the impression that Stewart was acting feeble. I saw him on TV a few times (Graham Norton, the Oscars, maybe something else) around that time, and he looked and moved way better than he did in the show.

Season 3 was way better.

I watched "Boston Legal" because of him. He clearly was enjoying himself, including making snide remarks about himself and making references to Star Trek. I wish him many more happy years.