For me, there were two "aha" moments. Both as I was hitting puberty, around 11 or 12. One was being at a public pool and a very fat MILF (well, my perception of what is very fat has changed since then, so maybe she was not that big) bumped up against me and boy, did I like how her soft fat leg felt.
The other one was when I was taking a computer class at the library. (This was way back when PCs were just starting - might even have been a PET computer we were learning on, for those of you who remember those.) There was just me, my sister and this other girl in the class. I remember that it was all I could do not to stare at her chubby thighs the whole time we were in class together. Fat legs have always been my thing, and this is the first time I remember thinking how much I liked chubby girls.
Memories...