You must be referring to professional sports and not college sports. If not, college kids make their own decisions when they are recruited at under the age of 18?
At least in California, they can't make the choice to purchase alcohol or cigarettes until 21. The more the science links football to permanent health effect, the harder it is to sustain the logic to let them play as "consenting adults". Say nothing of the lengths to cover up some of these effects by the NFL and probably the NCAA.
In the case of kids getting recruited at <=18 years, I think the question isn't necessarily about letting them make their own decisions, but whether this is a decision they should be presented with at that age. It sucks that physical maturity comes well before mental maturity.