Hyperparameter tunning for Random Forest- choose the best max depth
In general, the max depth parameter should be kept at a low value in order to avoid overfitting: if the tree is deep it means that the model creates more rules at a more detailed level using fewer instances. Very often some of these rules are due to chance, i.e. they don't correspond to a real pattern in the data.
Overfitting is visible in your graphs from the quite high difference between training and test set performance. One can observe that the performance on the test set increases with the first few values of depth (I'm guessing until around 5 or so), and it starts decreasing after that. So the optimal point (performance and no overfitting) is the point that you mention, it's after this point that the model is overfit.
The stable part of the graph is probably due to another parameter (e.g. min. number of instance per leaf) which stops the model from overfitting even more. If the model was free to overfit as much as it wants, it would probably reach max performance on the training set and very low performance on the test set.
Note: I'm confused why you're using both accuracy and MAE, normally the former is for classification and the latter for regression.