How many of you do you think you have to be a certain age before you decide that your only going to be one gender over the other gender?

Just wondering.
I believe that you should know by high school. You don't have to be open about it if you don't want but know what you are.

I'm just being told i have to be in my 30's to know what my sexuality is. Which i think is wrong.

What do you guys think?