It's hard to say I can remember when I briefly saw a girl topless on a Florida beach when I was about 10 and liking it, and also remember watching the scrambled porn channel at my grandparent's house (had to watch it there since we didn't have it at my house) probably a year or 2 later so probably about than.

But I can also tell you that I didn't start Masturbating on a regular basis until much much later.