[personZ]

But it's still true, in general.

At best what you could say is true is that people with no relationships and no "extra-curriculars/external interests" have more free time.

This crosses all ages. There are young people with filled social and activity calendars (I've worked with young people who seemingly had filled social calendars at work). There are older people who have all the time in the world.

I remember reading once that the best employees are the unhappiest (in life). These were the people with endless time because they had no activities, no family or social relationships, etc. They'd happily burn the candle 120 hours a week because they had nothing else to do. Not sure how beneficial that is to knowledge work.