My youngest friend moved out of her parents' at 19 and 8 months (not including friends who had children young) and at the other end of the spectrum my boyfriend is 27 and 1 month and hopes to move out sometime after the New Year.
My one friend moved in with her DP after they were together for 5 months and they are now happily married with a 2 year old. Other friends have waited years.
With marriage I think you need to have been with someone 3 years before you think about marriage. I think it's still sort of a honeymoon period before then. Same with getting married young. I sort of think you shouldn't get married before 23 because you just change so much in your late teens/early 20s.