Teenage girl issue with family? Am I overreacting? Please help me!! :)?

I am a very mature 14 year old high school girl and still have to share a room with my very immature 10 year old brother. I liked sharing a room with my baby brother when I was in elementary school, but things are different now. The door has no lock, and my parents won't let me put one on because they say I would lock myself in the room all day and it wouldn't be fair to my brother because it is his room too. I always tell him "Please don't come into the bedroom, I am changing" but he doesn't listen, or he forgets, or he really doesn't give a crap. Recently I have been changing in the bathroom most of the time because I am so sick of him walking in on me! This wouldn't be so bad if he would cooperate a little with me, but he is mean and immature and thinks it is funny when he walks in on me. I never have privacy, time to be by myself to just read or listen to music and get away from this kid. That never happens. He is a nightmare. He wrecks the room, makes a mess. If I leave money or valuables on my dresser he will steal them. He will dance around the room holding my bra up to his chest and giggling, trying to make me mad. One time, he found a box of my pads and he started opening them up, taking them, saying "What are these, what are these?" This whole situation is awkward and miserable for me but my parents never listen. They love him so much, he is their little baby, and they don't understand why I wouldn't want to be around him constantly. And they are planning to keep this arrangement until I go to college. This will be even worse when he hits puberty. I am so mad, I have considered moving out. I have a couple friends who have big houses with spare bedrooms, and they would be happy to have me. Am I overreacting? Would you be comfortable with this if it were you? Are my parents being unreasonable? Am I? Thank you so much for the help, I will be so grateful!!!