How do you know if you want kids?

I am a few years shy of 40. He is approaching 50. We've been together 8 or 9 years. When we met, he wanted kids and I didn't. Later on, I think I became more open to the idea of kids, but he had decided he didn't want them.

He just came home from a visit with his nieces and nephews and says he wants kids. Multiple. This is the first time he has mentioned this in years.

I have no idea how to process this. I have never had a burning desire to have children. At certain points in my life I have been open to and intrigued by the idea. Now it feels like a 'now or never' point considering my age - and this could be a deal breaker in terms of our relationship.