with Yamato about to be revealed (i think), there is 2 possibilities
1. Yamato is adult man (20 - 25 y/o), and he might be the "ACE" of the Beast Pirates.
2. Yamato is still a kid (around momonosuke age), and he isn't Ace of the Beast Pirates, that would lead to KAIDO being the "ACE" of the Beast Pirates.