This site says that you need to be 18 to enter Canada without a letter from your parent!
Any person under the age of 18 and traveling alone should carry a letter from his/her parent or guardian authorizing the trip. Travelers without such documentation may experience delays at the port of entry.
http://www.amcits.com/entry_ca.aspThe US embassy in London has this info:
"The Visit USA Association, a private non-profit group offers a selection of information on U.S. travel on 09069 101020.
(Warning: this is a premium rate number only available to callers from the U.K. Calls are charged at 1 pound per minute.)"
But inductively, I'd think that if you're 18, then you're good to go. No drinking or gambling until you're 21 and you might find it hard to rent a car.