Forget physical exercise to maintain eternal youth.
No longer age 12-22, but still going through puberty in my late 60s, I find one way to keep my memory active is to try to remember names of friends/colleagues from the past (high school, college, early employment).
When I (something of an insomniac) try to fall asleep by not by counting sheep, I seek to remember the name of persons I knew decades ago.
In the morning I (or my genes) succeed in identifying the right person(s), unfortunately not most of the time.
The "Eureka" moment of remembering the name of someone you knew decades ago can be quite exhilirating/rejuvenating (names forgotten but "miraculously" remembered, even if some of the persons in question disliked you no end).
I would judge this mental exercise healthful. But of course I leave it up to the "mental health" experts (or, to use the more au courant word, the mental health "community.")