I wouldn't consider 15 starting too early and I can understand that at this point in your future IT interests you wouldn't want to narrow yourself by choosing a specific field.
Basically the starting point for most would be learning hardware support. A+ certification is pretty much the starting point and a prerequistite for many computing courses.
It will introduce you to hardware, interrupt requests, resolving conflictes etc. An excellent starting point. before you get too deep ni C++, let me point you to Python. Resources are easily found by a ismple search, Guido Van Russom's text is an excellent learning tool. You will learn how to write a program in a few lines that equals hundreds of C++ lines of code and it is MUCH more secure and easier to debug than endless lines of redundant and not so useful code. Python programmers make easily double or triple what a C++ programmer makes and in much less time. Source code and modules are free, as Python is open source. Most financial institutions, secure payment methods and encryption/Punblic Key Infrastri=uctures utilize Python, BIG MONEY!! I hae a friend who got a two year head start on me with Python, he has earned over 3.6 milion in the last three years working part time setting up PKI and bank security systems. C++ is gettng old and too buggy to be viable anymore in these environments, not to mention it takes forever to debug and search through thousands of lines of code instead of a hundred.