I know it's a SIDs risk to sleep bub on their tummy, but until what age?

I struggled to get Alexzander to have a sleep this morning but finally got him to sleep on his tummy. He's on the lounge & I am right next to him so I can keep an eye on him, but I wonder if he can sleep this way in his cot yet? He rolls over now from tummy to back, but hasn't quite mastered back to tummy yet - he gets stuck halfway!