Ask HN: What's the right time to launch a startup?

Situation: I have what I believe is a good idea for a startup. The target audience would be unlimited, anybody could use the product.

Background: I just started my studies this year (18 years old), and I fear that if I launch a startup, I will have no time for studying or other important experiences I would normally have at this point of my life.

This video validates these suspitions (I think you all know it): http://startupclass.samaltman.com/courses/lec03/

Question: So I wonder if I should decrease work by concentrating to a certain, far smaller target audience. The product may then not be worth a startup, but rather a side project, that will also probably not pay off.

So here are the options I currently see: Should I waste no time and implement my idea, should I reduce the target audience or should I just wait for the right time for my idea?