This is a conversation I feel like I have quite a bit of experience in having been diapered 24/7 since I was 11. Not usually me having to defend my diaper wearing, but my mom's putting up the defense for me. I have heard "she seems a bit old to still be in diapers" or "isn't she like 8 or 9, why is she still using a diaper?" It has usually happened at places like the park, or at social events that we are invited to and people don't know about my situation. Now please understand, it has never been much of an embarrassment that I still wear and use diapers, I have never really shied away from it, and have always been open about it (not that I just go around the neighborhood with only a diaper and onesie on). So, for me, running up to my mom on the park bench as she is chatting with another mom and saying, "hey mommy, I need a change," isn't something that I would avoid saying. In fact, I have done it many many times. This either usually garners a strange look or sometimes one of the above-mentioned statement/questions.
Now I get that I am not in the same situation as most everyone else on this site, and for me it just appears like an older "kid" who is still in a diaper, and not a full-grown adult wearing and using diapers openly. But even with my situation, it is not acceptable for a kid my age to still be in diapers, and people have hardly been shy about it. It's just a different form of underwear, at least in my opinion.