Joshua was old means that his degree had worked itself out. He could no longer be a leader of those desires that would now be revealed. This means followers were needed.
Joshua does not have the strength to cope with the desires that are revealed now in the land of Israel. Therefore, he was no longer able to be in the place of Moses, as he had been all these years. He conquered as many egoistic desires as he could, corrected them to altruistic ones, and he cannot advance farther.
A person feels that there are such desires in front of him that he cannot lead them. He feels that this is not for him, that he is an old man, and this is not his job!
Nevertheless, the work must continue, there must be the next generation. So it is, generation after generation. What is meant here is not a change of a person, but a change of system of how to move forward.
Let us say that Joshua worked on level one of egoism and now we need to work on level two. He does not understand and does not know how to work with it. This is not his nature.
This is called that he has aged because he is not ready for this. The only solution is to pass it on to others.
From KabTV’s “Secrets of the Eternal Book” 8/2/21