Seriously, my personal theory of lovestyles is that there's a spectrum for poly-to-mono, just like the Kinsey scale for hetero-to-homosexuality. Some people are so far one way or the other that the far side isn't an option-- even if society happens to condition people the other way. Others are more moderate; they could go either way, and so what they do winds up depending on who they meet and how their community reacts and what they consciously decide is a good idea.
The experience that comes with age can play a role, but I don't think it's a primary determinant. Be who you are, and enjoy it.