I don't think that explains much, if anything. Of course some voters die, voters of all ideologies. But young people become old enough to vote and every voters age by four years between each presidentail election.
And many younger voters who'd voted liberal for a time get hit with the reality of real life and eventually become conservative, thus adding to the conservative voters.
The makeup of the voter population isn't static at the older or younger end. There's always some movement in and out of all political parties.
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.