Sorry I didn't actually reply to the OP so I will now.
I don't think there is a certain age you have to be, to be on either site. I actually made an account on TGR recently and I'm only 17. It's all about what type of skiing you enjoy and it's almost a maturity type of factor that goes into it too.
I can definitely see myself getting more mature as time goes by. I've also started to drift away from park skiing to as I've gotten more mature and almost have gained more common sense.
Actually from experience, I've gotten 8 concussions over time from motocross and park skiing. I progressed a lot in both and paid for it with brain damage and learning issues.
Now, I can still function and do daily tasks fine, but I'm actually no longer allowed to hit jumps and rails anymore, just because the risk factor is higher and if I get anymore head injuries I could be screwed up for the rest of my life more than I am now. (I still sneak in the occasional rail here and there though)
So right now I'm actually at an awkward point of my skiing life too. First, I live on the east coast, so there isn't much backcountry skiing here to begin with, especially this season. Since I was pushed away from park, I still wear all the baggy clothes, and ride center mount skis. All I do though is cruise groomers and fly through the woods, if conditions let me. I'm having more fun flying through trees though than I ever had any other type of skiing. Granted skiing in general is just fun, but I think I found my part in it.
It's just all about doing what you enjoy, and you can be on either site that you enjoy too. I love newschoolers for the humor on it and just the format is a lot easier to follow than TGR because it's what I'm used to and have been on longest. I like TGR for the more mature side and all around stoke on skiing.
Sparknotes: I do what I love in skiing, and you can be on either site. It's just what you love to do and everyone on both sites shares the same love of skiing.