H.C.
Hi! I don't think there is a right or wrong age to make the switch. I think it depends on the parents and the child. We got bunkbeds for my step son and my own son to share. My son was 19 month's and he did just fine. He's never fallen out of the bed and he loves his bed. My sister had my niece in a twin bed as an infant and she also did fine. She wouldn't sleep in a crib. So she took a bed from a top bunk bed and extended the gard rail and my niece would sleep on a bed and she never had any problems. I also have a friend whose son preferred the crib over the toddler bed and would choose to sleep in the crib even though they had a toddler bed set up for him. He was 4 before he really started to sleep more in his bed than crib.
I believe it's really up to the parents and child. Anytime is a good time to make the switch as long as you are ready for it and take the time to make it a possitive experience instead of negative. One thing we did is we let my son choose his very own sheet for his bed. I showed him a few different ones and then he started to ask to see some more and he choose his own. That also made him very excited to sleep in his big boy bed.