Personally if I am going to be totally honest, I never really grew up so have always had a like for diapers/nappies.
But if I where to pick a time when I first became aware that I wanted to be back in them post potty training it would be when I was 6 as my younger brother was a baby and I felt like I was being left out, as I wanted to be a baby again.
The second time was probably in the 1990's when the TV was bombarded with adverts for diapers and every time mum& I went shopping at the supermarket we usually past the baby section and yes it also had diapers for youths/adults too.
Although it was never something I made public knowledge of I just kept it to myself as I was too embarrassed to admit that I had strong desires to be back in diapers although I never went through with it.
Although for years after this I would sneak extra boxes of tissues and make my own diapers although they never really scratched that urge to be back in the real MC Coy so to speak.that was until I disecoverd I could fit into adult sized diapers although this was in response to a by then very real and genuine need to be back in them as I have covered before in different times.
I hope this helps awnser your questions.