Before doing some more research I had no idea that there were rules out restricting what age you could enter a pro sport. I have found out that in the NFL, NHL and PGA there are age requirements and restrictions. The NFL says that their restriction protects young people from physicaly harm.(Bianchi)
I'm going to say that I believe that there shouldn't be an age requirement. For some people, college is their thing and for others, sports is. For all you know, the next 18 year old athlete could be your next favorite player. I would like to believe that you only need general knowledge that you learn in high school to be successful in sports, but I'm sure there are people that will beg to differ.
It's somewhat difficult to find more information on this law, but I sill have a couple weeks. Hopefully I will have more luck tomorrow or whenever.
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Bianchi, Greg. "Age Requirement in Professional Sport." US Sports Academy (2006) 28 04 2008