The art and design training really doesn't have any pre-requisites in terms of age or prior experience. It is pretty accessible to just about everyone, although it was not designed for young children. We usually say that ages 12 and above can do pretty well with the material, but if you are not sure, check out some of our Blender and 3ds Max videos on YouTube to see how good a fit it is for you (or your child).
If you simply wish to learn some basic programming in C/C++, there is no specific age minimum or subject matter pre-requisite, other than a basic comfort level working with computers and getting around the Windows OS. Generally, computer programming with a language like C or C++ is usually not tackled until junior high school at a minimum, where high school or even college are the much more common cases. So, just bear that in mind.
It is not tremendously difficult to learn the basics of programming in C++, and we've seen some pretty smart young kids do very well with it, but if you are a parent, we usually say that ages 13 and up is also a good rule of thumb to work with here.
If you are a bit more serious about building games, we can start to get a little more concrete about pre-requisites. If you are pursuing the programming track, then you'll at least want to be comfortable with high school level mathematics (algebra, geometry, trigonometry). If you took these subjects earlier in life and are a bit rusty these days, don't worry because your membership includes some material to get you back up to speed. You don't need to be a human calculator to build games, but there is no getting around the fact that game developers have to deal with math. Everything you see on that screen is the direct result of mathematics happening behind the scenes, so there is simply no avoiding it. So, given these minimum pre-requisites in terms of subject matter, we tend to recommend a minimum age of 15 or older in these cases. Again, we've had some smart kids quite a few years younger than that do quite well with the material, but they are not the norm.