One of the problems Clarke had was that he was made captain at a fairly young age and younger than several of the older players like Hussey and Katich.
Kim Hughes had the same problem when he was made captain at a young age in the 80s ahead of older guys like Marsh and Lillee who resented him for it.
Some older players don't take too kindly to being told what to do by a captain that's younger than them.
I don't think age comes into it once they've established credibility as a player.