You don't have to go in costume! Just show up and take it alllll in!
I was ... hmmm ... 18 or 19? ... when I went to my first convention. Actually, I was a member of the San Antonio Star Trek Fan Club, and I worked the convention.
You experience your love of science fiction in a whole new and intellectual way at a con, hanging out with the like-minded and the completely weird ... often in the same person.
You should check out Confluence 2004, to be held July 23-25, 2004, in Pittsburgh. They're going to have a writing contest! Go check it out! http://trfn.clpgh.org
There'll be a lot of the younger set there, because a writing group for teenagers is having their convention at the same time and place!
It sounds like it was made for you!