The reason you are nervous is because at 14 years of age, you are in the adolescence stage. This is when you begin to realize yourself and how people are attracted to you. This causes you to be self conscious towards other people, and especially of your opposite sex. It is this factor that causes you to feel nervous around people.
Three reasons. 1, it's the age of puberty. And it really has a big affect on it. 2, because you're getting older and becoming more aware and understanding of how people judge one another so you become nervous thinking they might judge you. 3, when we're young, we don't care what anyone says or thinks of us, so we're never nervous.