Most toddlers have the ability to hop over the crib rail when they are about 35 inches tall and between 18 and 24 months of age.
Of course, some babies are particularly agile and will attempt to climb out sooner (at which point they should be moved to a bed), while less adventurous types will not try to climb out at all.
If your toddler is comfortable in his crib and not a climber, then it's okay to let him sleep there past the age of 2 -- just as long as you're mindful of his safety