In which year did the ottoman empire reach its peak and then begin to decline?

The Ottoman empire was at its most powerful and influential around the time of the reign of Suleiman the Magnificent and just after so this, so it would be between 1565 and 1570. After this from around 1571 it began a slow decline.

Answer;
In the year 1600 the Ottoman Empire reached its peak and then began to decline. 

Explanation;
The Ottoman Empire was one of the greatest empires that lasted for several hundred years. Under Suleiman the Magnificent, the sultan of the Ottoman Empire from 1520 to 1566, the empire reached its greatest peak of its territory and power. After several decades the Russian power began to grow and Ottoman naval supremacy was challenged by Europeans, notably at the Battle of Lepanto in 1671. Therefore; by 1600 the strength of the Empire was beginning to take a downward trajectory.