I recently came across this subject on quora.com and this is my reply.
Honourestly speaking to answer your question I was in diapers/nappies on and off right from the time of my birth in 1980, right until the age of 9 & a half in 1989/1990, this was due to bed wetting and daytime accidents.
However the first time was out of curiosity more than anything else this was from the ages of 6~9 not related to my bed wetting although I did love the feeling of wearing them again and regressing back to being a baby/toddler.
However the second time was shortly after turning 30 in age, when I started wearing them again
this happened right out of the blue without a warning when So because partially incontinent due to factors that where beyond my control or even desire and the fact that I believe my body was saying that I should never have been out of them in the first place.
I am now 40 and a adult who wears diapers/nappies from time to time and who actually enjoys them, as they provide me with happy memories of my childhood years and means I avoid embarrassing accidents.
Hope this answers your questions.