Well currently he’s what 22-23 and PC is what 28. They won’t give it to Green for a long time to come if they do but say Pat is 35 when he retires you would imagine in 7 years other candidates have arrived on the scene and because of workload (batting/bowling)Green will be a last resort. Currently Marnus and Head I’d have way before Cam.
Well PC is and will be captain for the time being but he's injury prone, has lost his good boy image by showing he's a hypocrite with certain cricket sponsors. He lost a lot of people on that front, they've allowed former captain, Steve Smith to fill the void for the games he has missed.
Travis Head would be the logical choice but he's 29 so he wouldn't be captain for long and Marnus is best left to play his natural game and not a captain IMO. Great batsman and part time spinner but he doesn't have the temperament to be captain and he's 28 as well.
Cameron Green is 23 and wouldn't be the youngest Test captain if he were to be selected. Ian Craig from the '50's was and still is the youngest Australian Test captain appointed at the age of 22.