I used to think that it was only for little kids (so don't feel bad) but I changed my mind a while ago.
Now, if you show up at my door in costume I'll give you candy. I don't care if you look 65, as long as you don't look like a creepy child molester, I'll give you candy. I give adults with kids candy if they are dressed up!
I have friends who have older kids who are disabled and don't mature in the "normal" (cough) fashion. Let them trick or treat, I don't care how big they are. All are welcome at my home.
I didn't get it together fast enough this year, but next year I'm doing the Teal Pumpkin thing so those with allergies know I have "safe" items and/or candy.