I'm 18, which is the advlt age. But I guess that can still count as a 'teen'.
It might not feel like it now, but 18 is VERY young. What it means to be in a relationship with someone will change drastically for you over the next 10 years if you're anything like most people. That's not meant to be condescending. My perspective on relationships has changed gradually but drastically since I was 18...as in, not even anywhere close to being similar. There is no age threshold that, once crossed, causes a person to magically become the person they're going to be the rest of their lives. Legal advlthood says nothing about where you're at or where you're going emotionally.
There are no rules as far as how long you're supposed to know somebody before you explore a romantic relationship. If it feels appropriate then try it out and be mindful of the feelings of others as well as your own. Waiting some arbitrary length of time is a good way to miss opportunities.

Then that means the time I spent with her was wasted time. Right?
If you enjoyed it or learned something from it then no.