The challenge is not people's inability to learn new skills but employers unwillingness to accept the risk of hiring mid-life new-learners.
This is only an issue if either there are genuinely not enough jobs in the field lots of people are trying to go into, in which case people should not be encouraged to go into it; or if older people demand higher compensation for the same skillset as younger people based purely on age, in which case that doesn't seem fair anyways.
And realistically, a mid-life learner is generally the better option to begin with - it's nearly impossible that someone after years of working has picked up no transferrable hard or soft skills and made no network connections of any value.
That's just a polite way of saying "willing to work for less" which again means either somebody's asking for too much or someone's being exploited, which is the real issue.
Obviously its mere anecdote, but every company I have ever worked for has gladly hired middle aged entry level employees. If anything they have been preferred.
Why is the burden placed on the employer to grow their employee and not the human being to grow themselves? In a generic sense, it could be outside of the purview of the employer to consider the employee's future in that specific way (in other ways it makes more sense, like health and happiness while employed).
It seems oddly paternalistic, to me, and once you remove that moral presumption doesn't the conversation become more complex?
Even if we did presume that jobs should be required in society, how would it even work for a job creator to preemptively know what skills were needed with enough time to ensure at your current job you were trained for future jobs? Why would that fall to your current employer?