Keeping them held snug against the body over a lifetime, reduces sagging near the end.

Interestingly, this was thought to be the case for breasts, as well, but up to a certain size (at which point support becomes necessary) its best to leave them be other than specific activities like exercise, because it forces muscles and skin to tighten to support them which keeps them from sagging longer. It’ll still happen, of course, either way.