Take myself for example, I started doing programming seriously around 2001 when PHP/MySQL was the hot technology. This meant that I got a lot of experience quickly in a field that was high in demand for new projects. This means of course that I now have several years of PHP experience and could get a PHP job relatively easily.
However very few new cool startups etc are being built using PHP but that is not a problem since there is still plenty of PHP around, however what happens in 10-20 years when all of the PHP codebases have been retired and everything is done in NodeJS or whatever.
I will be competing for jobs with recent graduates who "came up" using Node and who will be willing to work for lower salaries. I may have more experience than them in programming but probably not in something like Node. Sure I will be able to learn these skills on my own time to an extent but probably not with the sheer enthusiasm and vigor that an 18 year old who is dreaming of making his million dollar idea could muster.
Of course there are areas like algorithms etc that do not change so much over time, but the odds are that a bright college grad will be able to remember his CS classes much more clearly than an older developer would (unless the older dev had reason to exercise those skills often during his career).
The main thing an older developer would be able to offer is Wisdom, but this is largely something for which there is not a high value placed in modern software.