Each to their own, but I find the idea of children (SN excepted)much past the age of 2, in a cot with sides up, odd. I don't know anyone with a 2.5-4 yr old, who is still in a cot.
My DTS's (now adults) went into their own small, low, single beds at 14 months. It was earlier than I envisaged, but the large cot which they shared became knackered, and my parents treated us to the beds. There were some issues initially, with one or both getting up once put down, but the return to bed was swift and repetitive, and they soon stopped the "game".
DD is 15 months and on the diddy side. I would imagine we will convert her cot bed into a bed around 20-24mths.