Really early for me, I was probably 5 or so when I first felt "funny" about the idea of gaining weight.
I remember being fascinated with Hansel and Gretel, and wishing I could be forced to eat cakes and pies, etc until I got fat.
Also, I recall going to my first "all you can eat" buffet... I was probably 9. I put away WAY more food than I was used to. I felt like I was going to burst, and loved it.
Still, other than rarely stuffing myself when I could, the whole feedism thing took a back seat in my teen years. I remember cautiously Googling... sorry, Yahooing stuff in high school back in 1996-2000, but it was risky. I found out there was a book called "Dr. Chocolate Milk's Get Fat Quick Guide" or something, but there was not much online that I could find.
Once I moved into a dorm room, and had my own computer (this probably made the biggest difference), the feedist stuff came on a lot stronger.
One thing that made me mad, and probably made my roommate mad too... I was always ordering pizza, eating junk food, ice cream, etc... trying to pack-on pounds, but never gained weight. He went to the gym a lot, but had a weakness for the aforementioned food. Saw me with some, he wanted it too. And he gained the Freshman 30 in the first semester. All my efforts, and he was the one who gained. Dammit.
But yeah, I've "known" since I was 5, but wasn't really active with it until I was 18 or so. Even then, my weight held steady into my late 20s.
8 years