Your age is irrelevant unless you are renting a car. I'm 40+ years older than you, yet don't doubt that you could teach me a thing or six.

You don't mention if you have another Linux machine available to you. Get one. You'll have an easier time learning about the underlying operating system from a desktop than from the N900, and you'll eventually have to suffer the SDK anyway. Debian is your first choice (Maemo is built on Debian after all) and Ubuntu is your second choice (also built on Debian and n00bie friendly).

People here keep saying "Python". That's a useful skill, and you'll get results from it quicker. Distributing Python apps on N900 is problematic, but for your own device it'll work just fine.

C++ is a can of worms, and you can easily get lost in the forest. I wouldn't recommend it for anybody's first programming language. (That said, if you become a serious developer on the platform there is no way you'll be able to avoid it.) Just be aware that C++ has a high activation energy. When you start dabbling in C++, you'll be obligated to drop back to your Debian desktop because that's where the SDK is. You'll have to learn about the GNU toolchain, a nontrivial undertaking by itself.

So my recommendation is: install Debian, learn to drive around the Linux filesystem, write a handful of Python toy applications, install Python on your N900, run your toy applications on it, then start playing with PyQt. That'll give you a solid background when you graduate to C++, the SDK, the toolchain, and the Debian package system.