College athletes sports betting is illegal no matter the age, at least in Iowa but probably everywhere too.It is only a concern of states if the student-athletes are breaking any legalities. Gambling is legal at 18 in many of the states. If what has been rumored is correct (underage betting, using another's ID) those would not have happened if gambling was legal at 18 in Iowa.
With that being said, yes, many student athletes are illegally gambling per the NCAA while being legal gamblers through their states.