Disclaimer: The following are existential questions that do not reflect my own beliefs in any way, shape, or form. Why do older people deserve jobs? Why does anyone deserve a job? Are there instances where we as a society accept people NOT deserving a job? How different is age as an attribute compared to: Felons, the mentally impaired, the under-qualified, those lacking physical attributes?
For that question to work you need to explain why age would disqualify somebody.
A felon has committed a serious crime in the past, demonstrating they may be unsafe or untrustworthy.
A mentally impaired person may not be able to think things through and solve problems that come up in day-to-day work.
By definition, an under-qualified person can't do the job.
A person lacking a physical attribute may literally not be able to perform the required tasks.
There is nothing like that directly associated with growing old. In fact, experience is a big positive, and that actually increases with age, which should make older people more desirable than younger ones.