They make diapers in all shapes and sizes. You can't be too big, too old, too young, too.. anything really.
Diapers are a preference. Sure, society says otherwise and there is a stigma attached to wearing a diaper, but not only does no one need to know, but no one is trying to find out either. If you'd like to wear diapers, simply wear one beneath your clothes and makes sure to keep others in consideration by practicing the best hygiene possible and maintaining your privacy.
A common road block in the ABDL community is the fear of people finding out or noticing. Believe me, the general public is not looking at you or skeptical of you in any way - That is, unless you are trying to get them to notice, which I believe isn't right. Family and Friends are the group in your life that are more likely to notice and ask. If you aren't prepared to explain yourself to them, then I would be very careful about wearing diapers around friends and family.
Either way, to contradict the playground bully outlook of the modern world, diapers are simply just a different form of underwear. Even if you weren't planning to use them, they are perfectly acceptable to wear beneath your clothes if you choose to.