One of the most obvious–yet somehow easy to forget–facts about comparing elections over time is that age cohorts age. Today’s over-65s were yesterday’s 40-64s. So when the voting patterns of an age “bracket” change, we’re not always talking about the same people, and unless you think age trumps all other voter characteristics, there’s no particular reason a given cohort should behave politically just like its predecessors or successors.

That becomes very apparent from a nugget of data offered by Harry Enten at the Guardian:

[T]hose who turned 30 over the past four years have maintained their Democratic allegiance from 2008. The conventional wisdom is that people become more conservative as they age. This isn’t borne out in the research, and 2012 is no exception. The only age group whose vote increased for Obama from 2008 to 2012 was 30-39 year-olds, as those who had formerly been 25-29 years old moved into 30-39 year-old age cohort.

That Obama did unusually well among under-30 voters in two straight elections is a very good sign for the future of the Donkey Party, as we are already seeing in the slow upward movement of pro-Democratic sentiment among age categories. That’s another problem Republicans ought to be grappling with as they seek to minimize the ideological adjustments necessary to remain competitive.

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Ed Kilgore is a political columnist for New York and managing editor at the Democratic Strategist website. He was a contributing writer at the Washington Monthly from January 2012 until November 2015, and was the principal contributor to the Political Animal blog.