At the other end of the scale, I feel I'm only getting in to my stride after 14 years instructing and aged 69. Usually, the limiting factor is how long you can still get a Class 2 medical. Locally, one instructor gave up eventually at aged 86, still teaching aerobatics! Our examiner is 76 and still flying 5 days a week, though he's trying to cut back on the ab initio stuff and only doing the more interesting instructing, like tailwheel etc (the type he's got most tailwheel hours in is the Shackleton). I think Bill Ison was instructing up to age 88 at Cambridge.
TOO