Only 23, another NFL player is retiring early because of concussions
Chris Borland - Wikipedia
Bills LB Tarpley Cites 2 Concussions As Reason To Retire - CBS Detroit
Michigan football player quits over concussion concerns, Football
Chris Borland Retirement: Is Football Worth It?
Luke Kuechly Joins N.F.L.'s Under-30 Retirees Club - The New York Times
San Francisco 49ers Linebacker Retires at 24 Over Head-Trauma Fears
San Francisco 49ers Chris Borland Retiring at 24, News
Chris Borland Archives - Jeffrey Sterling, MD
49ers Linebacker Quits After 1 Season, Citing Safety Concerns : The Two-Way : NPR
San Francisco 49ers' Chris Borland reveals a concussion during camp made him walk away