How common is it for 20-somethings to live with their parents in Australia? Has this changed in the past few years?
I'd say "Very common". Most of my friends live at home. Two went almost straight to jobs from high school (after TAFE or a year or so of uni before stopping), I and a friend are uni students (yep, still, at 24), one friend is married, but if she weren't married she intended to stay home until after she finished her degree. My boyfriend lives in a shared house thing by 'himself', because he lived in the hills and it was too far to come to uni every day, and also he didn't get along with his parents particularly well.
People start thinking about moving out of home at about end of uni (so, 21 ish) but it's certainly not uncommon to stay longer to save money or because you simply can't afford to move out (I won't be able to until... at least a year and a half away.), if you're at uni or if you're not. I really don't even mind, I usually like my family! I also think that people don't want to fuss around with room mates who you might not like, move out with boyfriends or girlfriends you might not stay with, loneliness, deal with renting crappy places, and so on. And parents don't usually go "Right, you're moving out!!", at least not amongst my 'class' of people.
There are of course people who move out at 18 but I've noticed that is very uncommon. People don't go away to uni unless they can't get the degree they want locally, for example if they live in the country. Most Australians live in cities, most cities have universities, Tafes or other ways to have further education (apprenticeships or what ever) in your choice of career, so we just stay home. No dorm life for most of us.
I don't think it's actually changed much recently, I don't believe this is a 'Recession!!!' issue, this is a different cultural attitude and different geographical facts.
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