My daughter was 3 when we transitioned her to a full-sized bed (like a bit smaller than a queen). She was happy and safe in her crib and didn't try to get out of it. I was pregnant and happy that I had a sense of peace when I'd put her down for a nap and if I needed a snooze, too.
She also did great in moving from her crib to a bed in another room. We kept the same routine. We have a "gate" that goes between the mattress and the springs to help prevent her rolling out of bed. We never told her she couldn't get out of bed on her own, but she always waited for us to come get her up. Then, it changed to she waited for us to come open her door to let her know she could get up. Now, 8 months after she left her crib, she is just beginning to get out of bed to check the closet for monsters or get a toy she wants, then she hops back into bed. We recently had to tell her it was OK to get out of bed on her own to go to the potty; that she didn't need to wait for us to help.
Lastly, I know of a kindergarten boy that went to school, still sleeping in a crib. He told his teacher he sleeps in a crib and she "corrected" him with, "You mean a bed. You sleep in a bed." and he said, "No, I sleep in a crib." His mom was embarrassed (he wasn't), but she had a full house and didn't have anywhere else for him to sleep! So, sometime before high school is too old for a crib. :)