I really think it's a question of scale. If it's occasionally with a small group of known friends for short periods, that's quite different from roaming around by themselves or in a large pack for hours and hours day after day. It might also depend on what their friends are like and what sort of standards their families keep. I know that sounds judgy, and maybe it is, but 13 is pretty young still. The risk-taking part of the adolescent brain develops a lot sooner than the sensible part, and even if your kid is "sensible" there's a good chance at least a couple of the others are not. Maintaining a few parameters around the socializing reduces the potential for escalation of risky behaviour due to boredom. JMO but my DD is 14 and plenty of her friends are still socializing in semi-supervised ways, so other parents obviously feel the same too.