Rex,
Blimey’s figures for seniority are based on the seniority predictor spreadsheet that was created a year or so ago. It takes everyone’s age into account, but assumes retirement at age 65. It also doesn’t take standard attrition due to loss of licence etc. into account. Some years have significantly higher rates of retirement than others, but it only goes up as far as 2033. You cannot just extrapolate figures beyond 2033 to work out seniority in later years in the way you have done, because BA has some very definite age demographic variations at various seniority positions.