In my early teens, I was really skinny.
Late teens to late 20's - got into shape, no sport in general, just going to the gym about 3 times a week.
But in my mid 30's I decided to go through with my dream of gaining weight and being the fatty I always felt I should have been.
Took some getting used to in coming to terms with how my fat distributed on my body vs. the fantasy me I imagined, but I've learned to enjoy it and enjoy living a "fatsexual" life.