I will not dismiss the science premise because all players will eventually fall off. But when you hear of Crosby (Jagr, Kobe, Jordan, Bird, etc) doing excessive extra practice and work, and Mika unable to attend optional practices, I don't think science paints the entire picture.
Even with a substantial physical falloff, any 31-year-old should be able to take two extra strides to complete a back check.
So yes, he may have a valid decline, but he also looks heartless, gutless, and checked out.
I'd be sitting him every game because he's a black hole, contributing little and making almost everything around him worse.