I moved out for 3 or 4 months at age 19, but when I couldn't find a job, I came home (I moved about 1.5 hours away).
I moved out again a little less than a year later, at 20. I was going to school, but I didn't live there. I worked and lived at home to save up money to pay rent for my first year (saved up since high school ended, so for 2 years), and worked through school.
I moved back home two years later after spending 3 months hospitalized. Then I got pregnant, and decided to stay home. So now at 23, I'm living back with my parents.
I'm not sure when I'll move out again, but it'll most likely be before my 24th birthday. The living situation isn't bad at all, but I do want my own place again.
I'd say close to half of my friends between the ages of 20-25 still live at home. Two of my mom's siblings lived with their parents well into their adult lives (well, moved back home in their late 20s/late 30s, and stayed for an extended period of time).