That logic is flawed if you only account for age. Assuming longevity increases top out at 12 years, a 12-year captain is no more or less expensive than a 20-year captain. An airline needs X number of captains and Y number of FO's, the captains all cost the same (usually).

Put another way: Yes, the older pilot will make less during his tenure at the airline. But he will also produce less.