I'm curious for those of you who have kids who ski: at what age did you start to let them ski independent of you? By this I mean being in the lift lines, using the lift and skiing the trails by themselves or with friends? Any particular reasons for the ages your kids did this at?
DS attended ski days with his middle school when he was in 6th grade. He was 11 y.o. at the time. There was some teacher supervision, but I'm sure it wasn't very close supervision. He had already taken ski lessons though, so from a skill perspective, I wasn't concerned about using the lift or skiing ability. I was a little concerned about the other kids, who didn't have the same skill level or who weren't as responsible as he was.
If the child is able to ski, then my concerns would be similar to biking or riding public transit on his own. Is he responsible for sticking to the route we've discussed, is he likely to "goof around" and create a risk to himself or others, if he did run into a problem does he have the skills and resources to manage on his own and/or will help be readily available?
I think it would depend a bit on the area you're at. My kids were all skiing independently at 5-ish. But we are a small ski area and my dh is the manager. We are there every day, my kids ski every day.. so they learned to ride the lift at a pretty young age. But I do like my kids to ski with a buddy.. I don't think skiing all alone is a great idea.. although it happens sometimes at our small area. If we're at a large area I would insist on a buddy.