Where I live they are dismissed on their own as young as 8, and so long as they have a safe way home then a couple hours alone isn't going to hurt. There are enough electronic distractions at home now that they won't turn to delinquency.
15 is bonkers. There should be no need to keep your teenager on-leash for that long.
15 is bonkers. There should be no need to keep your teenager on-leash for that long.