He's 3 and been dry in pull-ups for a few weeks. We wanted to leave it longer until the warmer weather (to help with washing and drying!) but issue was forced last night as his tummy button has been getting irritated (bloody eczema!) and the pull-ups make it worse.
And he only bloody did it! 
I am, of course, well aware that having exulted in this success, he will then promptly have accidents for the next week or so, but this just feels like such a huge hurdle to have jumped! He's still not napping in my bed without a pull-up though