Does including your age, if you are a young author, give you a disadvantage?
Publishers select manuscripts by two factors: quality of the writing and marketability of the author. President Obama can write a very bad book and it will become a bestseller. He is a most marketable author.
Being a teenager (or even younger) is a strong point in marketability. There are many examples of fair to good books that became bestsellers simply because their authors were around 15 when they wrote them. A recent US-American example is Eragon by Christopher Paolini, which has been reviewed as a mediocre book but topped the international bestseller lists.