I am of the opinion that as long as both parties can fully consent, any age gap is fine.
So as long as neither is too young or mentally challenged, age is just a number that should be left to the couples discretion.
Auntie Ethel romance confirmed !

This goes double in a fantasy setting where different lifespans and age-modifying magics exist.
www.dandwiki.com/wiki/Ring_of_Alter_Self_(5e_Equipment)