Been doing some napkin math:

Retirements:

2021: 67
2022: 126
2023: 174

Aircraft Orders / Retirements / Net Gain
2022: 104 / 15 (estimate) / 89
2023: 100 / 30 (estimate) / 70

Additional Pilots Required (2021 hiring folded into 2022 estimates):
2022 - (89 * 13) + (67 + 126) = 1,350
2023 - (70 * 13) + 174 = 1,084

Currently we have 9100 pilots.

Hypothetical seniority of a very pessimistic 2014 hire today - 65%

At the end of 2022 - 57%
At the end of 2023 - 50%

By the end of 2023 we should be upgrading pilots hired in 2017, or 6 year upgrades. I'm going to assume if times are good, the most junior captain will 62% seniority.