D.K.
We moved my son because we wanted him to get up himself and go to the bathroom rather than calling us. He was 2 when he trained and was probably 3-1/2 when we switched to the toddler bed. Seriously - if he still fit in it (he is 6 now) he would probably still be perfectly happy in his crib. Never climbed out, never complained about it. He has been in a twin bed for over a year now and still occasionally falls out. We really only switched because he seemed too old to be in a crib.
We had a drop side crib. If I had to do it with a fixed side crib - he would have been in a toddler bed way way earlier.