I don't know what the easiest way to learn CAD skills is, but I suspect that they will be a common requirement for modelling before long.
I suspect that most kids - certainly those who are interested in modelling - will be learning the basics at school. But those of us longer in the tooth will have to do things on our own - much like the early days of R/C aircraft, when getting the thing into the air was a success on its own...
I found that an easy introduction into 3-d modelling was using a 2-d drawing package. Even something as simple as Paint will introduce people to the idea of having a drawing area, then using the mouse with various 'tools' to create line drawings - but I suspect that there is a better free 2-d drawing package available somewhere.
3-d drawing skills can then be added. Basically, these involve taking an enclosed 2-d shape and giving it 'thickness' - so a circle can be 'pulled up' to become a cylinder. This then becomes a 3-d object, can be rotated about its axes, and fitted to another object. Making several of these objects, modifying them and joining them together will create a complete drawing. Again, there are free downloads of 3-d drawing packages available, and it's probably worth learning on these before spending a lot of money on a professional product
The above might be a path that someone with no experience might find useful to follow...?