How old are babies when they transfer from a crib to a toddler bed?

In the Kaplan house it turns out 18 months, 3 weeks, 2 hours is the
perfect time. Our little Ethan lept out of the crib (head first) twice
in one night on Monday, 1-14-13. So now both babies are in separate
toddler beds. Yesterday and today Ruthie ended up napping with Ethan.
Seriously adorable.