It's actually about 1/2 price for the full price ticket. It's rarely worth it to use the youth fare since the regular discounted fares are usually better.
For example, a full price ticket is about $180. The youth fare would be about $90 but SWA's discounted fare, available to everyone, is $59.
You do not have to buy a "Youth/Infant fare" for children, you can buy a regular discounted rate for them.