I think its less about age and more about financial independence; the more monthly obligations you have, and total debt, the harder it is to be flexible in your career choices.

Have a low personal burn rate, save at least 20-50% of your income, and don't live in places with exorbitant living costs (SF, London, NY), and you too can be the master of your destiny.

This is what I had in mind when I described career flexibility. The experience for a middle aged person (say, late 30s or older) returning to the workforce full time after scaling back or leaving the workforce for 5+ years to take care of small children (and who still has those obligations for older children) will be very different form relatively young person who has gone back and forth between work and travel. I think the nature of the disruption to career progress is very different.