I love this topic
30 year old female, gainer who isn't exactly gaining at the moment (just thoroughly enjoying what I like to call the "maintenance plan"
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I stuffed my shirts with pillows and plush toys as a kid, too! I also remember a very strong need to eat until I felt stuffed from a young age, although I'd always attributed that to being an emotional eater...? I was fascinated by bellies, mostly pregnant bellies because I saw more of them than fat women. I remember feeling so fascinated by the softness of fat people that it embarrassed me.
I didn't discover feedism and weight gain as a fetish until I was 19 or so (thank you FF and *** groups!). From then on it's been a part of my sexual life (mentally, at least), though it waffles between the forefront and far into the background. I dabbled in gaining at 24, and seriously went for it at 29.