Yes kids aged 3-12 are in a room together, but they're not really together unless they choose to be. Theres so many options that they don't have to mix. While they're totally allowed in both spaces, younger kids are often attracted to the club and older to the lab. While the rooms might look more attractive to younger kids, as do the names of the programmes offered, once inside Id say the older kids are more catered for: iPads, computers, animation, the majority of the crafts, a lot of gaga ball/4square is played, the wii, card games, board games. Although a lot of 10+ are 100% sure they're going to hate the club/lab and want to age up, I honestly believe that if they go in with no pre-concieved ideas, they'll actually find a number of things that they like and enjoy.
But like I said, they'll be kids of all ages. There's a fair mix across all age ranges. Ive seen kids that are complete strangers become best friends within a few days.
I always tell the older kids (especially the girls), if they really don't have any friends, go and sit and talk to the counselor thats in the craft/animation area or pixie hollow, do a craft with them or play a game, and 99% of the time, another kid their age will come over and join in because they feel like they don't know anyone, and eventually theres a little group going and by the end of the game you know where everyones from, how old they are, who they're cruising with, how many cruises they've been on...