After more than 50 years in the instructing and examining business, the following observations of myself and others may be of interest. With advancing age, acquiring new physical and co-ordinative skills does take longer. Not a great problem if you accept it. I started skydiving in my 50's. Your handling skills also degrade, but to some extent are replaced by experience if you maintain regular practice, until I suspect they just fall off a cliff when it is definitely time to quit. Lastly, but probably most important is the progressive degradation of situational awareness, or the ability to keep track of and interpret a fast moving, complex or unexpected situation. On the ground, we just take things a bit more slowly but in the air this isn't always an option. Anyway the signs have told me it's time to quit as an FIE and FIC, but I will instruct for bit longer