Most pay scales top out at 12 years, not 20. And they don't reset when you upgrade. That's why most captains at the legacies make the same per seat. If you hired 60-year-olds exclusively, no one would ever reach year 5 pay, and you would have incredibly low cost pilot labor (not factoring in the incredible training churn.)

But a 53-year-old pilot will cost the same as a 25-year-old over a 12 year period (more if you factor in insurance.) Beyond that, the guy hired at 25 will be on max pay, but so is every other captain at every other airline. Hence, no advantage.

The true advantage goes to airlines with quick upgrades (usually LCC's and start-up regionals.)