Great question. I have a story about my expereince since we just went throug this. My son is now 3 1/2. I had the same question for awhile. When I asked my Dr at my son's 3 yr old appt, yes he was still in a crib, he kinda freaked out and said he was worried I was displaying the disease where you try to keep your children babies (??). I was like what on earth is he talking about. My son was NEVER a good sleeper, he occasionally sleep walks when he sleeps really deep and he started really liking his crib around 18 months. So we were all sleeping, safe. So I never considered I had to move him to a big boy bed for HEALTH reasons. When I explained I was the farthest from that disease, my son was off bottles when he was 11 months, in daycare, walked at 10 months, loves to be called a big boy and we constatly push him to be big and not act like a baby. Well, he recommended that we put him in a big boy bed asap. I took it with a grain of salt. My son has always done things on his terms when he was ready. I have found that the more I push, the more he refuses. So to make this as less dramatic as possible since my son Loved his crib, my husband and I started having open discussions about it in front of him, like wow he is so big he will need a big boy bed soon. Then one night, when I put him in his crib (he never once tried to get out) he hugged me and said I want a big boy bed now. The very next day he picked out his sheets, pillow (spiderman of course) and new bedspread. We never looked back. He now loves his big boy bunkbed. I am a nervous wreck with the idea that he could be up and running around. He does get up a few times, but nothing major. That was about 6 months ago.
So, I can tell you that you will get all kinds of reactions from peopel about sleeping in a big boy bed. I had so many people judge and look at me like i was crazy for letting my son sleep in a crib until he was 3. But it was his choice and I really did not care what people thought as long as we were all sleeping. We had no signs of when it was ready either. We told my son the option was there and when he was ready to let us know.
My point is to make the decision based on safety and if he wants to have a big boy bed. My Dr seemed annoyed ot learn I waited and let my son make the decision. But, he is very happy in his bed.
Good luck,