Help!'

I'm currently 19 and my boyfriend is 25, we've been dating for almost 4 years. I want kids someday but he keeps on bringing up topics about kids and asking me if we're going to have kids. I told him that I would like to go to university first and he just won't stop asking for kids. I don't want to be rude to him about it because he loves children, but I just want him to stop asking! How can I tell him to stop asking me for kids?