About 18 months
Around 18 months, a toddler can climb out of a crib by flipping himself over the railing.
A toddler bed, or just putting the crib mattress on the floor, is fine to use at that age. Parents must be sure the entire bedroom is childproofed and safe for a toddler to explore and play in without supervision. The toddler can have the whole room to himself at night (with a good gate at the doorway).