Where the GOP lost was among the age groups 18-29 (60% to 37%) and 30-44 (52% to 45%). They comprised 46% of the electorate. Romney won the other age groups but by a narrower margin.
My own theory is that the changing demographics of this country is what is fueling this disparity. By 2019 half of the children 18 and under will be minorities as defined by the USG. Each year a cohort turns 18 and becomes eligible to vote, it will be composed more and more of minorities who overwhelmingly by two to one vote Dem. Our immigration policies are driving most of these changes. The Dems are well on their way to becoming the permanent majority party.
We WON those groups...among white voters.
Our problem is: Minorities. There are just MORE of them. Especially in the younger age groups.
We don't have to WIN among minorities.. but, we can't lose 70-87% of them, and expect to win...EVER.