900 is arbitrary and not special significance to my knowledge.
A small shift in number would not dramatically change the overall story. Airlines occasionally binge on certain years of hiring but not so much years of birth. Example: Almost no one was hired in 2020 but thousands hired in 2022 - so the difference between a 2020 hire and a 2018 hire isn't huge, but 2022 to 2024 is. There isn't really anything similar with "Oh we don't have anybody who was born in 1965 so no one is retiring in 2030." Point being ... a young person hired in front of a hiring wave will be very senior for a long time no matter how you measure it.