Realistically, chances are that if you could come up with a good idea for a startup and successfully follow through before university, you will still be able to do it a few years later. Even if the startup world implodes, you'll probably just have to wait a couple more years. I would guess that the added education improves the odds for most people, and failing a startup with a degree and related career to fall back on its far better than the alternative.
Aside from university being fun, you should also be learning skills and meeting people. I'm working for a startup that started at my university and I heard about it through another student. If I decide to do a startup one day I'm guessing my experience and contacts will be an asset.
"Realistically, chances are that if you could come up with a good idea for a startup and successfully follow through before university, you will still be able to do it a few years later."
True but you also have to consider the people you know who might be able to help you. If your friends are leaving [1] to do something (and you believe in it) then that opportunity may never present itself again. Having close relationships with the right people you know well and can trust is important. You can't just establish that on the fly (think of any girl you have ever dated and how the relationship changes over time as an similar example).
[1] Or if it's your idea and they will leave with you.
Aside from university being fun, you should also be learning skills and meeting people. I'm working for a startup that started at my university and I heard about it through another student. If I decide to do a startup one day I'm guessing my experience and contacts will be an asset.