>>30921284I'm just being devil's advocate here anon, but plenty of places you can get married when you're as young as 16. And Lillie was their couch warmer, they didn't legally adopt her.
Kukui has to be older than Juniper though, either way. Even if he and Burnett got hitched at 16 right out of Professor School there's been at least some passage of time since the end of Black/White, Grimsley has visibly aged (arguably significantly). There's no possible way for Kukui to be younger than 18 without Dialga sneezing and fucking up the time line. And likely Guzma and Kukui are the same age if they journeyed together on their Island Challenge.
Conclusion, there's no possible, logical way for Guzma to be younger than 18. Even if Gamefreak SAYS he's 12 it makes no actual logical sense using their own fucking time line.