It isn’t purely a matter of race, the white percentages used to be a little higher, and Romney’s turnout was slightly higher than McCains’s, not 4 million lower.
I’m not aware of anything showing that the white youth stayed home in 2012.
I said that wrong, the turnout WAS lower for 2012, but Romney did increase his share by less than a million over McCain in 2008.
As far as how it broke down I don’t know, Romney did easily win the Independents, yet still lost, it might have been conservatives who stayed home, possibly because of Romney coming out as pro-choice, and running pro-choice ads in battleground states.