Is there not a contradiction in the requirement to have a younger pilot in the cockpit if a mature pilot is in command of the aircraft? Ageism works both ways.
At the age of 65, perhaps it is time to leave commercial transport flights and perhaps return to training and passing on those hard earned survival skills to a younger generation. A quick survey through the last couple of decades worth of AAIB reports indicates that older pilots are involved in considerably more incidents than younger ones. That may well be due to many taking up flying as a hobby in retirement, but there is a clear correlation between age and accident rates. Much the same pattern can be observed in vehicular accidents around the world too.
A blanket cut off age seems to be the easiest way of trying to keep flying as safe as possible for passengers. It may not be fair to individuals and particularly those endowed with better than normal health, but it is a system that has worked well for decades.
I learned more flying a few hours instruction with a mature retired commercial pilot than I did flying tens of hours with a 22 year old hours builder. They could both fly the aircraft and instruct, but the mature instructor knew how to demonstrate finesse on the controls and when and where to push the aircraft to its limits. It isn't always about avoiding mistakes that can get you killed, it's dealing with the consequences and maintaining control of the aircraft to ensure a safe outcome. That is what maturity and experience brings to instructing.