Mel Gibson is too "old"? WTF? He's not even 55 yet. What's the matter here--they can't think up any kind of interesting adventure that concerns Max in his middle-age years? That's pathetic.
Maybe Gibson and the filmmakers wanted to avoid the recent trend of 80s-era movie heroes coming back out of retirement for "one more adventure". We got it with Rambo, Rocky, Indiana Jones, John McClaine, and it is looking more likely that it'll happen with the Ghostbusters as well.
Perhaps, they wanted Mad Max to stay young.