How Old Is Too Old To Trick-or-Treat?

My mom told me to today that she had a trick-or-treater in the 20+ crowd today.  My mom felt she was too old to be trick-or-treating.

I remember when I was around 8, my mom commented that the our neighbor (a different one) was trick-or-treating as an 8th grader - she felt this was too old.  I made sure to make her eat these words.  I think I trick-or-treated at the beginning of high school - and I know plenty of my peers did.  I overheard some people talking today about trick-or-treating in college.

What are your thoughts?  How old is too old?