Once upon a time we mocked the Soviet Union for its gerontocracy. Aged party leaders, bundled up in overcoats and fur hats to the point of near-unrecognizability, would be wheeled out to sit, immobile, as parades passed or party congresses opened. Their withered, stale leadership was emblematic of the decaying USSR’s withered, stale ideology — and industrial base. But now the joke’s on us. A leading United States senator, Dianne Feinstein (D-Calif.), can’t seem to remember important things, like her just-taken vote on a judicial nominee and her just-announced retirement. “Did I vote for that?” she asked an aide Wednesday,...