That’s not a plausible theory to me. IME older people are just less likely to work hard. They bring a “rest n vest” mindset. If an older person rounds a 40hr week down to 35 hrs, and a younger person rounds up to 45 hours because they are passionate and ambitious, all else equal the younger get person is better ROI for the team. Obviously this is stereotyping and may not be accurate but that sentiment can explain many decisions.