User blog comment:Kaido King of the Beasts/Chapter 891: Pudding is Hella Underaged/@comment-53539-20180113201330

Political marriage was common place in the Pirate era. Marriage could happen any time from 10 years of age. IT depends on the context of the series and not in contrast to the real world.

Also bare in mind Big Mom ruling over her islands, means there is no rules, and she likely could marry off her children at any moment she pleased.

Oda used to be able to only draw the "Nami-body" type, he got a lot of complaints about it only on. Pretty much in OP as soon as girls hit puberty they bloom out into a woman. Kuina is about the only girl we have to go by from the "about to bloom" stage. It can be presumed any girl after Kuina's age at time of death quickly grows into the Nami-body type.

Never less this is why this body type is common, it took him ages to figure out how not to draw women like that and Amazon Lily is pretty much when we see his diversify the most.