Natural ageing does not play a large role in either game, unless you spend an inordinate amount of time in jail. If you do, I don't believe that there is any way of undoing that.

What you need to worry about is unnatural ageing from spells and attacks by ghosts. In both games, there are potions you can drink to reverse the effects of magical ageing, although in MM6, since there is a fountain of youth, there is a significant penalty. As for natural ageing, again, you are SOL, but you almost have to be doing it on purpose for it to matter anyway so...

Now, in some versions of MM6, ageing is broken and does not have any effects at all. But if you use a patched version or the GOG version it does make you frailer, as it does in the rest of the games.