I'd speculate (although it's not really speculation, just common sense) that the age group that has, percentage-wise seen the largest increase in the past 20 ye

I'd speculate (although it's not really speculation, just common sense) that the age group that has, percentage-wise seen the largest increase in the past 20 years are those born ~1980-2000, so ~21-40. The oldest of those folks were just getting into the workforce 20 years ago. Those in younger age groups aren't (at least not in large numbers percentage-wise) in the workforce yet, and the older age group had already been in the workforce. QED.