There is a key moment in a democracy and one that marks its maturity. While many people focus on George Washington's decision not to seek a third term as President - which he would undoubtedly have won - the retirement of John Adams was more significant. Adams did seek another term - it would have been his second - lost, and walked away. If Adams, Commander in Chief of the American military, had not decided to accept his enforced retirment, the whole history of the US, and possibly the world, could have been different.
I think this was the first time a political leader had ever had an Adams moment. Athens elected its generals, and even impeached some, but its political leaders were chosen by lot. Rome elected its magistrates, but they were precluded from serving a second consecutive term, so no serving Consul was defeated in a bid for re-election. Britain's Prime Ministers did not change as a result of general elections until the nineteenth century.
Saxon and Polish kings - like medieval Popes - were elected, but invariably for life. The question of re-election did not arise.
So I think John Adams was probably the first. It is a key moment in history, and his contribution should not be overlooked.