"Some of our worst losses occurred through defective leadership on the part of a unit commander, who might lead his pilots into a trap or be caught while climbing by an enemy formation approaching out of the sun. During periods of intense activity promotions to the command of fighter squadrons should be made on the recommendation of Group Commanders from amongst Flight Commanders experienced in the methods of the moment. If and when it is necessary to post a Squadron Leader (however gallant and experienced) from outside the Command, he should humbly start as an ordinary member of the formation until he has gained experience. Only exceptionally should officers over 26 years of age be posted to command Fighter squadrons."
Air Chief Marshal Sir Hugh C. T. Dowding. Despatch submitted to the Secretary of State for Air on August 20th, 1941.
My italics. In this case it seems Dowding recognised that age-gained experience can be detrimental; a little on-the-job retraining was required and an age limit recommended to stop the influx of older, stuffy officers leading squadrons.