It doesn't matter if it was "about" 20 years ago in one statement by Rayleigh. It's clear as day that SW was given to be 22 years before the present timeline if not outright stated. It doesn't matter what Rayleigh generalized, there's no way Hatchan was 16 in SW like you seem to think he was.
First off, SW took place 24 years before the present time line, Roger died 22 years before Luffy set out on his adventure. Second, Hachi doesn't need to be 16 in that flashback. He needs to be 8, which he easily could be. It's never stated anywhere that he's the same age as Arlong. There are 2 pictures that Arlong is probably where he seems like he is the same age as Hachi, and they're both silhouettes.