As Lloyd Grossman used to say "Let exaaamine the evidaaance"
Most UK airlines have a seniority scheme. So they know the age of the pilots when they employ them and they can predict the age at which they will retire.
If they have a good grasp of the market and understand the lead time to bring on to line new equipment then predicting crew resources is mathematics.
So why has there been a cyclical surplus/shortage since I was a Nipper?
BA have suddenly decided they need 80 bods to fuel their workforce. Surprise surprise.
HR - doncha jus luv em