Outside of extremely rare triage situations, this limit is true for all developers, regardless of age or quality of output. Beyond about the 6 hour mark, people can force themselves to sit in a chair and hit keys on a keyboard like the Swedish chef from the muppets, but it's pure management denial to believe those hours count as "productivity" and in fact when the defect rates start to get high and fatigue compounds and morale drops, it's really anti-productive.
Really, younger age is just usually correlated with fewer extracurricular responsibilities, i.e. home repair, health maintenance, domestic responsibilities, marriage, children, etc. These things tend to increase with age, making it a scheduling impossibility for older workers to sit in the chair at 8 pm going "hur duh dur" while slapping keys.