Read this on wiki like a week ago
http://en.wikipedia.org/wiki/Boomerang_Generation
Fwiw, in my experience i moved out on my own at 18, then back at 19, before moving out again at 20 where i now live (im 23 now) in Manchester a fair distance away from family. I also have 2 friends from school who live in the city who moved here 2 years before i did.
I also have a friend who moved out at 18, then back into his parents house at 19 when he dropped out of uni where he still lives. He is also a total underachiever - always had the best results of our group at school but now works part time stocking shelves in a supermarket (i kid you not). He doesnt look to be moving out soon. For a really smart guy, he is probably the biggest retard when you consider the years he has wasted, and that is a lot coming from me who has drifted around aimlessly since forever it seems.
Then i have a friend who has always lived at home, but has a pretty good job working in IT - he will move out sooner or later, but chooses to live at home mainly out of finacial needs.
House prices here in the UK are stupidly high - my generation will always struggle to get on the property ladder - i only have two friends from school who own their own place - one moved into a place with her boyfriend and doesnt have any money to do anything but pay the mortgage and stock the pantry, and then there is a friend who bought with his (then) girlfriend and he is in a pretty crappy financial position because of it short term (despite him earning the most out of all my friends).
There are a few others who have gone through similar routines - end of the day my generation the rules are just different, we cant buy unless we wanna be crippled and renting is REALLY expensive back home due to a lack of new housing. Manchester its a lot easier - there is more new housing and with a large student population the rental prices are kept down by this demand for cheap no frills accomodation.