40 is about right. By that age they have accrued some useful skills and have a settled family life maybe. With 20 years of possible service they have enough time to identify successors and cultivate their talent.

And some festivals need an army of very able sergeants and corporals. They should equally have appropriate talents. I well remember Andy Stafford at Toweresy did his customary round of the ceilidh tent and looked up as well as round. He spotted the wind had snapped a joint. He organised ladders tied it up immediately. They cancelled one workshop, held the second event on the grass outside and by mid-day the inspector had arrived from Bristol and declared it fit.
Same weekend, Shrewsbury suffered in the high winds too, but they lost more than a day's dancing. Some skills are not obvious, and rarely needed, but when they are - it can tell. Like looking up!