It depends on your style. If you are worried about growing out of it later but still like lolita fashion, starting with Aristocratic style might help. It's more of an 'adult' lolita and any age can wear it. But you can do whatever style you want.
I'm 23 and I'm not really into the sweet lolita. I'm more gothic, ouji(boystyle), ero, and aristocratic and I can see myself wearing them when I'm older. It's just a matter of preference. If you adore the sweet loli I don't think it matters what age you are. If you love it, go for it.