Welcome to the club, I'm also 18 and in college. For a lot of us saying we "knew" at a really young age is a massive oversimplification. I for example have three brothers, and spent the majority of my high school at an All Boys school. In both these environments It was easy for me to be the most feminine and so even though my body dysphoria was enormous, the social dysphoria- the one I have felt in myself from a really young age- almost disappeared, it didn't but almost...
I probably knew at 5- when one of my dad's business friends came for dinner and kept using female pronouns and everything, it just felt so good

As soon as got into University though the social bit hit me REALLY hard, I shut down and was absolutely miserable, one of the things I remember is just realising how much I was thinking about transitioning and coming to the realisation that if I wanted it to stop I had to give into it.
Just because you can't remember coming to the realisation at a young age, doesn't make you any less trans*
Also, Gay and Bi are different to trans. Trans people can be gay, bi, lesbian, pan, a or whatever else. trans* is an identity thing and although it is linked with sexuality, they are completely different things.