I remember very clearly. I was in grade 1 and was at an assembly when it just came to me that wearing diapers would be a fun and exciting thing. I never acted on it until 9 years later when I got my first package of diapers (depends).
At about that time, I remembered that when I was 6, I was punished for wetting my pants by wearing one of my sister's diapers. So I guess that's why I am interested in diapers.