I was either 14 or 15 when I actually masturbated.
For literally years before (probably starting at 12) I had been "into" crossdressing, wearing a pad in my panties, and wetting it. I did that for a few years, staying up late enough that I could try to look up videos of girls peeing themselves or wearing diapers on the shared family computer.
I've been really fixated on wearing diapers since I was probably 3 or 4. One of my earliest memories is getting caught having stolen one of my little brother's diapers, and my dad asking if I would rather be wearing those than underwear. I lied when I answered 'no'.
I remember renting the same VHS tape about astronauts, just because they describe having to wear Diapers on takeoff, and during EVA trips and moonwalks. For a few years after that I wanted to be an astronaut.
At about 7 or 8 I had some little booklet that came with one of the 2000s Scooby-Doo direct-to-video movies that had a picture of Scooby Doo boys underwear, and Scooby Doo pink panties. I remember that I kept that booklet in a drawer by my bed so I imagine wearing the panties every night for a long time. I was just a kid so maybe it was only a few months, but I distinctly remember it.
Eventually by the time I was out of middle school and in highschool, I knew how other people were "supposed" to masturbate, but I had already spent my entire 'sexual awakening' wearing panties and wetting my pants. Eventually I tried it the way other people described 'jacking off'. It was really unnatural. It felt good, but at that point I had already spent basically my entire life wanting to wear diapers. It wasn't fulfilling in the way I really wanted it to be. I've always felt this sense of "You know deep down you're not like them. You belong in diapers. You wear panties. You're inadequate as a man."
It's just the way I've been from the very beginning.