When did you put toddler in big boy bed?

I have a 4.5 yr old and my other son will be 2 in June. My 4.5 yr old we put him in a big boy bed a month after he was 2. He did great we had no issues. When should I put my other son in a big boy bed? I feel this one is alittle more tricky because he shares a room with my oldest. So hes definitely already trying to sleep in his bed and not his crib. But im nervous of him getting out to play.