Final felling takes place when the trees are between 70 and 120 years old, when the diameter of the trees is sufficiently thick. But other factors should also be considered, such as the risk for damage and your own financial situation.
If you harvest too early, when the trees are still growing, you will lose future growth and revenue.
If you wait too long, the growth rate of the trees will slow and the risk for damage increases, which can entail that your forest loses value.