I'm working on the theory that If you let them do stuff independently and actively teach them how to do so (safely) then they're much better prepared for when they _have_ to do stuff. It worked for me as a kid, anyway. Travelled about on buses within about 20 miles from about 11 or 12, and trains for local-ish day trips/inter-city visits to relatives from about 14, I think - I certainly remember doing the train York to Newcastle, metro to haymarket, bus to Blyth to visit my Granny at 14 with a school friend, but might have done it earlier. Left with older sisters to fend for ourselves fairly regularly from the age of about 13, since I had a part-time job so couldn't go on family weekends. There were a lot of parties and unsupervised sleepovers at whoevers-parents-were-away from about 15.... First ever trip abroad was the school French exchange at 13, followed by repeating that at 14 and 15. Going away for weekends visiting mates in far distant places like That LonDon or Shropshire from about 16. Went on holiday to Wales with a bunch of mates and one big sister when we 15/16 and big sis 18. Buggered off to live in the south of France for about four months when I was 19. I think it probably helped that I had a fairly big group of thoroughly decent friends and my mum was confident in us all looking out for and after each other.
I'm a little bit cautious with the EldestCub, since his Aspergers can make some situations atypically challenging for him, but he's actually getting pretty adept at life for a just-teen, on the whole. The SmallestCub (nearly 8) has declared that he is never going to leave anyway...