Regardless of if people get more conservative as they get older, the two-party system encourages both parties to make subtle adjustments to their platform over time so that neither will ever lose or gain much in terms of the percentage of voters they get in each election. Look at how the parties have evolved on the gay marriage issue for one example.
But that's assuming that each party is controlled by rational and self-interested people.
The open question is, as each party takes more and more control of the selection of candidates from the elites and gives it to ordinary voters, will there be a set of voters that doesn't understand their own rational self interest, and steers their party into the ditch, and breaks the two party system?

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