Definitely a misconceived perception of the two - people just look at the numbers next to age and assume "drop off" in play.
Incorrect.
McCaw has certainly lost a step or two. But he's made up for that in size, and has adapted this with a different type of play (rather than trying to make every breakdown, he picks his moments; and also added a more punishing running game and is still excellent in defence).
Carter is still a master playmaker. Maaaybe lost a step, but that was never his game. He's definitely the closest thing rugby has seen to a genuine Quarterback - choosing lines and which runners to put into space, and into which space. Analysing the game well, and knowing just when to run, pass, kick or spread it wide.
He's lost nothing there, if anything has gained more with age.
In saying that, I am not against a Cruden/Carter five-eigth combo with Smith at Centre. Especially with the ABs game being more about using the backs as precision line breakers (save for Gear/Savea), and using our loosies as the battering rams.