You are confusing length of service with age of employee.

To be fair, these aren't unrelated since they track together over time.

This article doesn’t seem to account for stock compensation which can greatly incentivize staying at a company for a long time

I randomly picked the very first article I could find because I was on my phone, but it remains true no matter what. In the very best case scenario your company's stock explodes over the (typically) 4 years of your vest, but after that it will be adjusted down, so you will be earning less than new joiners again.