And is it age-related? (Forget discrimination here, this is medical)
All depends on the individual I think. Whatever your age if you are overweight and/or take no exercise or do not use the rest period sensibly then you might be more susceptible to fatigue/tiredness. There is always the question of nutrition - how proactive are the airlines at providing their crews with healthy nutritious food whilst on duty? With the company I work for I have to take my own food to work so its down to the individual to ensure he/she has adequate food/drink whilst on duty.
In summary, I think there are many factors which affect the level of fatigue and I hardly think age is that relevant. Older pilots are usually more experienced and able to see the "wood for the trees" which might arguably mean they are less stressed and therefore less susceptible to fatigue.