I left DS1 in his cot as long as possible, basically until it was dangerous not to move him, i.e. he was climbing out, so around 2.5years.
He fell out of his bed for a long time, we did the matress on the floor for a few months but hed just sleep on the mattress not the bed so we got rid of it. His cot converted to a bed though, so it was rather low to the ground, but then you can get single beds that are quite low as well.

How high is your single bed.