Like we talkin thinking as in learning about reproduction or like jerking off? if the latter then probably like 10-12 or so. idk. I remember watching those soft-core porno movies they'd play late at night on TV
I started exploring how I could make my penis feel good at a very young age. The first specific event I can remember happened when I was 5-6, though I seem to remember seeing pictures in family albums where I fiddled with my crotch at age 3-4. When I was about 7-8, I first fell in love (not fleeting childish love, but one that I didn't let go of until I was 14-15, and I still think about), and started thinking about penetrating the girl I liked. At 9 years old, I started properly masturbating to the idea of penetrating her, ejaculation and all.
About 8. I recall getting distinct urges to hump girls from behind, though I had no idea why.
Started rubbing my dick for pleasure at about 9-10, and played a great deal of some "show me yours and I'll show you mine" around that age.
Got my first dry orgasm at 11 which came as a big serendipitous surprise.