When do you think it's the right time to ask your boyfriend if he wants kids?

Me

I don't mean to start trying but I just want to know if he wants kids because that would be a deal breaker for me if he didn't. We've been together for about 4 months now, we recently exchanged 'I love you' for the first time but I've been concerned that maybe he doesn't want kids because he's quite a bit older than me. When do you think it is appropriate to ask?