I went through since 2000 and this will sound very vague for you, but the answer to all your questions is that the groups are very mixed. Varying age ranges (I was 25 at the time), various employment backgrounds with some folk straight from uni and others that had good jobs and not so good jobs. Ratio of girls to boys was about 50/50, but then this differs course to course, as do all the things that you've asked.
Whatever you think the group will be like, you'll probably be wrong, but you'll be pleasantly surprised I'm sure and you'll make some really good friends. There are normally a couple of weird ones that are there as well. Be kind!