It's possible that it's not so much age that is the issue - it's more to do with the role they play. There are certain New Gods who seem eternally youthful - I'm thinking particularly of that assassin guy who dresses like he's in the Renaissance; can't quite remember his name - and others who seem eternally old or middle-aged. My theory is that they age normally until their role in life has become clear, then they rapidly turn into an appropriate approximation of that role. Therefore, Granny Goodness is old, yet powerfully built, as is appropriate to her role as insane parental figure; Desaad looks like Peter Lorre's sadistic cousin, that assassin guy I mentioned is eternally youthful, suave and limber, Verman Vundabarr looks like what you'd get if you put the love-child of Hitler and Napoleon into a wine-press, etc., etc.