Probably about 5 or 6. I discovered at a young age that putting pressure (and one time, a vibrating back massaging device) on my penis resulted in an "Awesome, tingly feeling". Honestly, I'm glad to see so many other people caught on to it so early, too - I always thought it was weird that I discovered masturbation so early! The first time I ever consciously masturbated (I didn't even realize my earlier behavior was masturbatory at this point) was around age 12, although I didn't actually cum until I was about 13 (cumless masturbation is
way more convenient in terms of tissue and sock consumption

).
Been a proud daily masturbator ever since - as a result, I don't even like receiving handjobs from my girlfriend. Or any girl, really. I have yet to meet a girl who is better at it than I am.