A doctor isn't going to take legal action against you because you and your same-age partner are having sex. But like someone else said, they may tell your parents if they feel it should be a concern of theirs, especially if there is a health problem. Honestly, though, I wouldn't worry about it too much. It's really important to be going to the doctor if you are sexually active, so I wouldn't let those fears deter you from getting proper health care.