Sold a hunting license to a 17 year old today. He couldn't remember his address (dad had to help*), had problems remembering his phone number. When it came time for him to sign it, he had to print his name (of course) and dad had to remind him to write in his last name also.
I swear the kid was not stoned, and seemed to (otherwise) be of "average" intelligence.
This isn't the first time I've seen similar problems from kids around 16 to 21 years. I don't expect much from those that are younger than that.
*They've been there a while. Dads drivers license had the same address and was issued 3 years ago.