All joking aside, I think part of this is just perspective. Once you get to 30 you look around at your peers and think “30 doesn’t look so old.” Same with 40. One of my students once told me “old is your own age plus 15.”
Nevertheless, I do think we take better care of ourselves now than people did two generations ago, in part because we know better.
Everyday life has changed too: many physically demanding manufacturing jobs have gone overseas in the past 25 years, so most of us don’t have to endure the kind of daily physical wear-and-tear our grandparents faced. My grandmother worked in a silk mill and my grandfather lined the hulls of ships with asbestos (guess what killed him).
Medicine is also better now. Nothing will put the years on you like an un-treated or under-treated illness.
So part of it is definitely environment, self-care, etc, but I think perspective plays a role as well.