I would say 18 or 19 years of age, because at that age, your pretty much leagal to do a lot of things, and also you should be old enough to respect those around you, not to shove, swear at little kids cause your a little older to know better how rediculous that is, and also at that age, your old enough to know not to talk to strangers, and its more unlikely that someone is going to kidnap you.
So as long as the 18, or 19 year old is in a group of friends, and they all have a cell phone, I think that should be alright.