I'm not sure if the original poster is still around (there have been a lot of comings and goings over the last 3 years!). However, I can give an idea from a friend of mine whose 21 year old son came out a few months ago (the friend emigrated 4 years ago, but the son stayed in the UK with his grandmother to finish his schooling). He's obviously no longer in school, and not yet found a job, but has got in with their local football club, and is having a fantastic time - he's made loads of friends.
So - really - the best thing is for them to pick hobbies - if you are staying in an urban area, rather than rural, there will be at least one club for any interest (as long as it isn't *really* specific). On the whole, everyone is really welcoming (to be perfectly honest, I've been more welcomed and 'clicked' into the social circles here than in a couple of the UK clubs I was with before I left), and it's a great way of making friends.