As with potty training, there is no "best" age. You have to judge if your child is mature enough for the switch.
Some people have 1 year olds in a toddler bed and the baby does just fine.
Some people move their child and life becomes a nightmare of shouting to go back to bed, endless trips back to bed, and midnight trips to empty all the cupboards in the kitchen or open the front door.
There is no mental harm in leaving a child in a crib.
Most kids who try to climb out and fall don't try again. Those that do keep climbing out should be moved to a bed or a crib tent needs to be put in the crib.
Evaluate YOUR child. Will he stay in bed at bedtime or will he be up playing / visiting you all evening? WIll he nap or will he play? How baby proof is your home or are you willing to lock him in? The older the child, the more trouble he can get into but also the more he understands.
at 4:27 PM on Oct. 31, 2008