How does one know when it is right?

I am quite young, about to start college, and I am in a relationship. We have been dating for about two years and I feel absolutely devoted to this man and I am in love. We can talk about anything and everything and we never properly argue. I find myself thinking about marriage but then get scared because perhaps I am too young. In the fall we are starting different colleges and so our relationship will be long distance. Do you think I am foolish for falling so in love for so young? Are long distance relationships do-able or impossible? And finally am I utterly bonkers for wanting to marry this man so young?