Here’s what Oda said about it:

Question: I feel like One Piece has tons of characters that are old dudes, but is that just because it can’t be helped due to the nature of its story? Or is it because you have the “I’ll die if I don’t draw old dudes” disease? P.N. Uhocilia

Eiichiro Oda: I love me some old dudes! I feel like age adds a new sense of depth to characters, both male and female. One Piece has tons of really interesting old people! But in the real world, age tends bring people down to a weaker state… and I’ve always disliked how there are some young people out there who just can’t have a little respect for these aged individuals because of this. So with One Piece, I’ve brought to life one of my countless fantasies: which is for there to be super cool old people who’ve maintained their strength throughout the years.

One day, you also will grow old. It’d be great if all of you could remain just as strong as the One Piece oldies are!