Data I have recently seen says that 250 age related retirements expected through end 2020, as everyone seems to be going to 65 now.
Average age of all BA Captains is 50.2 years
Average age of the Captains in the top 1000 seniority places is 51.9 (so, on average 13.1 years before they retire)!
3610 BA pilots on seniority list of which 1700 captains
Do the maths!