Should there be age limits for Trick or Treating? Hell, no. It's supposed to be fun. Allow me to share with you my story and the reason why I don't impose age limits. Trick or Treat bullying:
Shoot, as long as you're in a costume, no matter how simple, I give ya candy. When I was about 8, I had a lady refuse me because she said that I was too old. I was a (nearly) 6'0 tall 8-year-old (I also hit puberty at 9, for those who're saying bread/breasts = too old), who went away from that woman's house crying, ashamed, and embarrassed. I don't judge. I don't care about your age or height. You dress up, you get candy. Simple. It's supposed to be fun.
We're not all cut from the same cloth. We don't all develop at the same rate. My kids are already tall for their ages! Someone refuses my children, you damn skippy I'm going to say something to them. Stop being a bully. It's just candy. It's suppose to be fun. No need to be a bitch about it and police who gets what due to looking too old. You don't know their story. Stop judging, especially children.
You know what, honestly, costumes aren't even important. As long as you're having fun, just hold out your hand! There's worse things a person could be doing than Trick or Treating!