I had one, about twenty years old come to my door, in costume, he had a couple of younger kids with him. Just a minute of talking to him explained the situation and I gave him candy too.
The two younger kids, about 10-12 years old were actually escorting him on the rounds, he was mentally about 8-10 years old.
You're right though, generally 14 or 15 is a bit too old.
They were his siblings.