I meant the oldest surviving institution. Democracy as a form of government isn't an institution in the sense of a particular regime. The American institution of democracy isn't the same as the ancient Greeks' form of democracy. There are some commonalities, but not enough to speak of a single, continuous institution. In any case, the Middle Ages separated them.

There may be a fine line between a practice and an institution. We inherited various ancient practices, but does that make for a continuous institution? It depends on the meaning of that word.