5 or so.
When I was a young lad, knee high to a grasshopper, my mother dropped me off at a friend to be taken care of. I was young, about 4 or 5. Occasionally, others would have their children put into her care. One day I was playing with a little girl a bit younger than me when she wet her pants. When the babysitter found out, she was put back into a diaper. Until that point, it never occurred to me that that was an option. I had my own accident a few months later; it was unintentional, as I never had the guts to do something like that on purpose. I remember telling her to not put me in Pampers, in the hope that she would. Alas, she followed my request. Maybe I should have kept my mouth shut. By the time I was 6 and had started school, I was trying on some spares my mother kept around after toilet training my sister. I had a good 30 minutes to myself. I sure didn't understand why I liked wearing them, just that I did.