As many others have said, these interests started to emerge for me when I was young. I think my earliest memory of wanting to be put back in diapers and treated like a baby again goes back to when I was 5 just going on 6.
As I recall, I was having a hard time adjusting to school, I was just starting 1st grade and I had some bullying problems. To make matters worse, my Mother was always hung up on me being a mature young man, even from that early age, it also probably doesn't help that I was potty trained at 18 months. Well during this tumultuous time, my little cousins were born and I recall being very jealous of them, they lived what I perceived to be the good life, they had no worries, they could do no wrong and their parents didn't scold them or force them to be mature, grown up or well-behaved all the time.
From that moment on, I began wondering what it would be like to be a baby again quite frequently, it even made it's way into my play routine. I would stuff my underpants to simulate a diaper and I would play with my toys in a very childish way. There was a group of neighbourhood girls who I would play with on weekends and they loved playing house, of course, I was always the one to play the baby. Whenever I watched a TV show or a cartoon that featured a character regressing or in a diaper, I would secretly wish that that could be me and I was sometimes wistful that such things couldn't happen in real life... or so I thought.
Strangely enough though, my wanting to be a baby and wear diapers petered out for a bit after the age of 9, I can't for the life of me recall why. It wasn't until I was 14 that these desires emerged again, it was also at 14 that I learned about the ABDL community and found that I wasn't alone. At 15 was when I decided to stop being ashamed and start accumulating supplies, I would buy store-brand adult diapers whenever I could and I invested in a nice stash of baby things. Of course, I was caught in and around this time, my mother found my baby item stash, but at that point, I had already resolved myself to the fact that this was a part of me, so her tirades and shame didn't sway me. I now enjoy being a baby whenever I can.