I can tell you from personal experience (I'm a physician) that the vast majority of adults are ignorant of the real risks of all kinds of activities. When I perform procedures that require informed consent, patients frequently under/overestimate various risks and frankly require a lot of hand-holding through the process.
It's asking a lot to expect a 21 year old to comprehend "hey, at 50, you might not be able to walk without a cane because of football."