It really depends on the room, it's usually as low as 4 and as high as 10. I would suggest that you ask how big the room is to accommodate a group of a larger size. I've been in some that are rather tight for a larger group, but you may give up comfort for brain power.

I took my 12 her old nephew and he was better at figuring out things than the adults! A mix of adults and kids is great. I think 9-10 is as young as I'd go, but it depends on how mature the child is.