All of my best teachers were 45-75 year old men who went back to teaching after earning enough money in the real world. When I was in middle school the old principal stepped down (She was in her late 60's) and the new principal, a 40 year old career educator, immediately stepped in and demanded the removal of all male teachers. She was fired within a year or two but by that point the damage was done, three teachers retired, and the guy who came in afterwards did nothing to even it out.

One single bad year damaged 20% of the school, and no amount of good years equaled any more than a 5% improvement. That's just what happens when doing good is passive and doing bad is active.