Am I being young and immature?

My boyfriend and I are both 18 and have been dating for 3 months. At a music festival this past weekend, he and I were sitting in the back of my friend's VW bus. He turned to me and said, "Can I ask you something?" Of course I said yes. His response to me was, "I love you so very much and I want to be with you forever. I don't have a ring," - I was laying in his arms - , "and I am not down on one knee, but will you marry me?" I said absolutely yes. Later the next day we were sitting under a canopy and he kneeled in front of me and asked again. I told him that I wanted to so badly, but that I would wait for him and he should ask me in 10 years. He said that he completely agreed and that he wanted to be able to provide for me...I am going to college in TX and he in TN - 1000 miles away. I have a medical career ahead of me, so lots of schooling. Is it silly to start planning a future with someone, when in reality I have no idea where I will be in 10 years?