I think Final Fantasy delves into some pretty 'adult' themes at times though it may come across childish because of the fantasy aspects of the game. In FFIX, Zidane has that superdeformed chibi look and sports a tail, but he's still gets pretty mindsmurfed in Pandemonium, discovering that he's not human and has a limited lifespan.

If you're after a Final Fantasy with a little less 'fantasy' and a darker, grittier, more realistic setting and overall presentation, I say, wait for FFXV.