Well, while there are a lot of adult themes, it could be that Final Fantasy doesn't present them in an adult way. Don't get me wrong, I love the series. But there are books on racism, genocide, rape, abuse, etc for children and teenagers. The content is just presented in such a way that it's easy to access and doesn't upset them, but still teaches them things they (might) need to know right now, and will definitely know later.

So while I think that there's a lot of adult content in Final Fantasy, I don't know if I'd say the game is really made for adults. I'd say the audience they most appeal to is late teenagers.