Chloe asks: "I'm 18 and my boyfriend is 31. He's ready to start a family with me, but I feel like I'm too young to even start thinking about having kids. I'm flattered that he wants me as the mother of his children but the thought scares me too. He's the only guy I've ever dated, and I really care about him a lot. How do I tell him I'm not ready for kids without him breaking up with me over it?"
What concerns me most about your question Chloe is the older man aspect of your equation. You probably already know, but dating someone who is thirteen years older than you at this stage of your life is a challenge at the best of times. He's ready to settle down and you aren't. He's already learned how to be independent and live as an adult, whereas you're just getting started - and may still be finishing high school. Basically, your goals and aspirations aren't going to jive with your boyfriends'.
This might be difficult to hear, and I realize its not what you're asking, but you need to tell him that you're not ready to have kids yet and prepare yourself for the relationship ending because of it. This isn't your time to start a family, and from what you've said, that won't change anytime soon. If he's dead set on having kids ASAP, then it might be best for both of you if things ended now.