I don't think it's a matter of having a "regime". I think you either stay young at heart or don't. Sometimes, in fairness to some who are not, health problems do make feeling youthful difficult. I kind of think, for the most part, people are often "old at heart" decades before they're 50 or 55, but nobody calls them "old at heart" because they have their chronological age that says otherwise. They start out old-at-heart. Their age catches up. And, there you have it - an old, old-at-heart, person. LOL When you're young and you're young-at-heart age does nothing to change you for the most part. So you end up old but young-at-heart. :)
I'm the same "me" now that I was decades ago, and while I know I'm a whole lot wiser now than I once was, I really don't feel at all any older (thankfully). Maybe the only thing I really consciously do is to refuse to have my age define me and dictate what I'll do or not do (and according to someone else's idea of what over-50 is). I define what being over 55 is for myself; and, to me, being over 55 means still feeling very much the same as I did decades ago. (In fairness to people with medical problems, though, there's a certain amount of understanding friends and family need to have. Living with a long-term/chronic medical problem has a way of knocking the young-at-heart right out of people after awhile.