didn't expect Killingham is a fishman hybrid, I guess the one with the mask on God Valley is his father or relative because he looks way too young to be there.
None of the Elders have aged at all, but Garling and most likely Sommers have aged.
But then you have Gunko who looks young and didn't age at all, and we know she is old because before Brook became a pirate, Gunko was a kid during that time.
So Killingham could be young or he could be really old but looks young.