This is a tricky one. You've parsed it wrong; the expression here is actually come down to, not come down.
As Macmillan says:
come down to something (phrasal verb) to be the most important aspect of a situation or problem : In the end, it all comes down to who wants the job the most.
So, in this article, the writer is saying that the most important factor in the upcoming election is the generation of the voters. It could have been rephrased using the verb hinge:
Although the majority of people in Crimea are ethnic Russians, the BBC's Mark Lowen in Simferopol says the outcome of a referendum is by no means certain – and may hinge on a generational split.
hinge (intrans. v.) (hinge on) depend entirely on : the future of the industry could hinge on the outcome of next month's election.
I can understand how this would have been tricky to find in a dictionary, though, because the words come and down both have multiple meanings, and are both used in a good number phrases and idioms.