What do I do?

So the worst but maybe best thing just happened to me the other night! One of my best guy friends asked me to marry him. The problem is that i do love him just not exactly in that way. He told me that I was the only person he could see spending the rest of his life with. Im 20 years old and hes a year younger than I, to me I believe that that is a little young to be getting married. Who ever reads this I would really like your input. Thanks -Hannah!