It is not a matter of planning, but of adapting to change. I have worked in environments where change occurred regularly (outside forces at work) and I saw it was the young who coped with this change best.

I would need a concrete example but I'm not buying your assertion. Some of the most flexible people I know are all developers, irrespective of age. If you told me that older developers were more likely to ask questions and seek evidence before making a change I'd believe that. I find that most people confuse easy adaptation to change with pliability due to lack of experience.

I've seen the exact opposite. Startup, pivoted 3x in 3 years. The younger devs have a much harder time accepting he pivot and figuring out how to adapt. So without any more substantive evidence, my experience definitely refutes this silly argument that youth is more flexible.