@KingsGenerals
The Ottomans strength came from location as the link between East and West for trade. As soon as it became more profitable for West to go around Africa, the decline was assured, even if it did not materialise instantly.
1492
(also 1488 when Africa was first rounded)
@KingsGenerals
I believe it was the Age of Discovery. The lack of reliance on traditional trade routes not only affected the Ottoman economy but the transfer of ideas and technology. Being left behind, the Ottomans were a boogeyman in the Med until Lepanto shattered that image forever.
@KingsGenerals
Battle of Zenta.
Because after that point, the one big advantage Ottomans had in being able to mobilize massive armies, would be kept in check by an ascendant Russia who also had manpower for days.
@KingsGenerals
Already said in the comments, but It's 100% the moment the West did not need to rely on the Silk route as much and went via Africa to Asia and discovering the Americas.
@KingsGenerals
I would said that Battle of Lepanto 1571 makes the Ottomans being stagnant while Battle of Vienna 1683 was the gate for entering their declining era.