The current maximum age for single pilot flying under UK MAA rules is 65. If a pilot has always flown fast jets then, generally, there is no significant reduction in flying ability with age and g tolerance is not affected. The issue is that if someone has a break away from fast jet flying then the older they are, the longer and harder it is to get back up to speed. The motor skills of controlling an aircraft do not tend to perish but the airmanship does. Also, technology moves on so there may be new technology operating skills to be learnt.
I know of a pilot who is over 60 and is currently flying 3 fast jet types under MAA regulation and gets current to fly a fourth type when required. This includes delivering high onset rate g training and flying aviation medicine trials at 9g. He has been flying fast jets for 39 years continuously other than a brief couple of years flying for an airline. As a child, he wanted to be a pilot when he grew up. He achieved 50% of this; he became a pilot but didn't grow up!
On a serious note, there is a difference between just flying the aeroplanes and flying on operations and above I am not considering flying in combat; that is a separate discussion.