Mental difficult task are physically exhaustings.
The thing with rallying is that it's not one stage, the crew is busy and has to remain focussed all day. Janvanpuura is not wrong, but he needs to call it exhaustion instead of stress.
It's like walking 80km, it's never not difficult because the speed's to high, it's difficult because it takes so long.

Being old is not a physical problem. The level of fitness required in the wrc can be achieved by a 60 year old. It's a problem of the mind. As you get older your reactions get slower, and there's nothing you can do about it.
For a codriver its doesn't matter that much, because they don't need to react in milliseconds.