Engaged at 18?

Long story short, I've been with my boyfriend for about a year now. We always talked about getting married, and I honestly do see myself staying with him forever. He's the best thing that's ever happened to me, he's like a best friend, and he's taught me so much. Last week he left for his basic training and he's going to be gone for four months with limited contact while I start college. 

Last week, four days after he left for Georgia, he called me pouring his heart out and crying. He asked me if I wanted to get married, just like we had always talked about, and I replied, "of course" to which he responded, "I'm serious, will you marry me when I got back?" 

I was completely taken aback and I explained to him that I want to wait until I'm at least financially independent from my parents, but I'd definitely be okay officially committing and being engaged until then. I promised him I'd work as hard as I can in the next two years so we can get married on paper at 20 and have a ceremony after graduation and his retirement. He then told me to consider it an official marriage proposal. 

I have two questions:

1.) Do you think this happened because he was so overwhelmed by the shock of moving away so soon and the military and what not or do you think he was really serious? He sounded absolutely serious, and, like I said, we've talked about it before and he always talked about his life plans and included me in them. 

And 2.) Are we too young? I feel like when I was younger, I always thought anyone getting married with less than 5 years was nuts, but if my plan works out, we'd be together for about 3 years before marriage. I feel like that's pretty reasonable. My mom actually tried to get me to leave him before he left because she thinks I could find someone better in college, but I can't imagine finding someone any better. He treats me so well and I've never had someone care about me more than him. Thanks for you input in advance, guys!