Under what circumstances can all trees in a forest be of the same age?

In principle, this cannot be. A forest is a natural community that has emerged over a long period of time. Some forests have sprung up for hundreds or even thousands of years. Accordingly, all trees cannot be of the same age. This can only happen when the forest is created artificially by man. A certain area is needed, where trees of the same age begin to be planted or tree seeds are used. But this, in principle, is not possible, and even if we assume that this happened, after a while new trees will appear.