When I was around 4, I started being interested in wearing spare pull ups for toddlers that might need them if relatives came over. Sadly I was easily found out because I was 4 and wasn't sneaky enough. My parents quickly suppressed my fascination.
I also had a large pack of diapers kept at the top of my closet for storage at the time and when my parents found out they removed the package.
Skip ahead about 8 years when I was 12, my interest in diapers returned. I was pretty small and could have fit into those diapers even at that age... Darn.