Grossmann plays a reckless style of hockey. Injuries go along with it.
His knees have been a recurrent reason for concern as he had been wearing braces left and right last year, he's been struggling with hip issues earlier this year, and now severe concussion symptoms. It probably all started last year after the Malkin elbowing when he came back early and appeared dizzy to a certain degree.
Players need to be more aware of potential long term issues and take time to heal. Age is a non factor - Bourdon is even younger than Grossmann and may be done for good due to similar lingering effects.