I bought my loom by advertising for several months on Craig's List, and purchased the hex kit for $5. My hubby made a square wooden one years ago from wood and headless nails. I think they're equally easy to learn.
As to your question about the best age to learn to knit, I think it depends upon the child. I know 5 year olds who can knit and other children - including my youngest - who needed to be 9. However, as it was child labor for most of the past 500 years, so I suppose any child can do it! :-) A friendly local yarn shop usually has learn-to-knit classes for all ages. There's nothing quite like having someone to show you the ropes. If you have a Knitting Guild, you may find they welcome beginners or offer beginner's night. My daughter and I have been guild members for many years, and enjoy the camaraderie of our fellow fiber enthusiasts.