This is something I was pondering yesterday when the Mater was waxing lyrical over my latest finished object.
I can imagine that you need to be a certain age to achieve a basic pattern - the reviews for the one above suggest about 10 years old is a good starting point. But then surely from there you simply need sufficient eye sight and time to complete a project? Scaling up to larger patterns is just a matter of spending more time on it. Smaller squares mean a greater degree of hand-eye-coordination is needed so perhaps you'd need to be 15 or so to do those, but after that, isn't it just putting crosses on fabric in the colour you're told to use?
One thing I did find difficult to work out before I sat down and read the instructions in this tutorial, was smocking.