This is true. The problem with the logic is that everything has an opportunity cost (i.e. if you're doing one thing, there's something else that you're not doing). So on the face of things, using this argument to advise someone trying to choose between a few mutually exclusive options is tantamount to saying nothing at all.
For example, suppose that in the near term the OP will either move to San Francisco for this startup or attend college.
If he takes this full-time job, he'll miss out on the unique educational and social opportunity that is going to college full-time.
If he takes this opportunity to go to college, he'll miss out on the unique educational and social opportunity that is working on a startup full-time.
Obviously these two seem symmetrical. I'd argue, though, that after going to college for four years, it would actually be easier to replicate the startup experience he'd miss now than it would be to replicate the college experience four years from now when he'll be "the old guy" in his freshman dorm. Unless he's Van Wilder, that may not be the best social situation in the world.
Of course, this is all a red herring if he's not interested in going to college right now.