The main difference between a long board and a short board is length. A long board is typically defined by being longer than 8 feet while a short board is usually somewhere between 5 and 7 feet long. The price is determined by so many things that there really is no average price.
Long boards are more expensive because they require more material to make.
I know kids who have learned to surf as early as 6 years old, and older people who waiting until they were in their 50s to learn to surf. There really isn't any age requirement. My only training is actually surfing. I try to surf everyday for at least a couple hours. It is possible to be badly injured while surfing.