They're the ages that they can cross over both the clubs for their age, and if you allow the 12yo to be a self-check then they are free to move between as much as they like.
They'll get a feel and know what they like and where they feel comfortable it could be that they both prefer edge or the younger prefers the lab and the older the club...
As for making friends, I shouldn't stress too much, if they do the stuff that interests them, that's the best way to make friends. From my experience they have ice breakers and meet you're Counselors and things like that on the first night in the club, but they're often during show/dinner time and it's pretty quiet..
I've seen kids not rock up to edge till the second day, they came because of the activity going on (e.g. Gaga ball), they're in a team and suddenly immediately make friends because they're in a group of kids that have similar interests. Especially in edge and vibe, it's a smaller team that works in them and they get to know the kids better and quicker and often 'put kids together' "hey Jonny, did you know jimmy was from the same state as you." "No! Where you from? What team you support?" Or they play the wii together because they're the only two that want to play that game. Again because of the kid - Counselor dynamics, the structured activities often mean that all kids get included and involved and part of a group even if they haven't had the chance to make friends yet...
Then in the club/lab, kids just seem to find each other. They see a kid with a ball "hey you want to play?" Or playing a card game they just join in, or they sit next to each other on the computers and get chatting.
Some of the best cruise friendships I've seen have come from kids that have accidentally found each other, not through the structured/navigated activities.
My advice for all parents, especially first time is don't try to predict your kids. You won't believe the amount of times that I've had parents tell me that their kids definitely will or won't do something (good and bad) and they've been proven wrong. A cruise (especially a first
Disney cruise) is probably unlike any environment that they have ever been in.