.... My rule of thumb is: if they can CLIMB out of a crib...they are too big for a crib.
This is my practice as well ... I generally start to transition to a sleep cot around 18 months and just set the expectation you stay resting on the cot.
Children construct their own intelligence. The adult must provide activities and context, but most of all must be able to listen. Children need proof that adults believe in them. Their three great desires are to be listened to, to understand, and to demonstrate that they are exactly what we expect."