L.S.
I was really lucky with my first. She moved to a twin size bed on her second birthday. We got the bed and intended to just set it up and keep it in her room along with her crib for awhile so she could just get used to it before she switched. But, that same night she insisted on sleeping in her new bed. We actually had to push the crib up against it because we didn't have a bed rail yet. We got the rail the next day and took the crib down and never had a problem. She never got out of bed and just slept through the night as if nothing had changed!
On the other hand, my friend had 2 girls. The youngest slept in her crib until she was 3 years old. At that point, my friend made a big deal about her getting her big girl room, picking out decorations, blankets, etc, and she took the crib down then. So I guess it really depends on the child.
Good Luck!
L.