Dear Adda, what is true for one country/culture may not be true in another country/culture. For example, it seems from your question that there are strict rules regarding titles for people of different ages in your country. This isn't quite the case in the United States or in Britain, as far as I know (speaking as an American).
It's quite possible that a young person in his teens would be addressed as Mr. in a preparatory school in the US. But there is no legal age when a male receives the title Mr. In official documents, it's possible that a male boy could be referred to as master. My grandfather always addressed letters to me when I was a boy as Master Kim Metzger. The same applies to females, ie. Miss vs. Ms. or Mrs. I dont' know why you never heard of it. But you can find master in most English dictionaries. Merriam Webster's - master: a youth or boy too young to be called mister.