What about the case where the old person (and of course this would apply to any age in a sense), can no longer function in their job - in other words does not meet the basic requirements of being proficient in their job responsibilities.
We had some one like that a couple of years ago where I work. She is a sweetheart and had work for the company I'd guess over 40 years. It put a burden on everyone else at a time when we were laying off people and understaffed. If you were paired with her working on a project, it was as if you had no help and had to take the job of two people.
No joke - this woman would sleep on the toliet at work. I went into the bathroom and heard snoring - then a little while later she walked out. No one had the guts to tell her to go - they kept saying just give her proficient on her review - she is going to retire soon. They were saying this for years before she finally did.
I do think the job gave her value and a sense of being worth while, but to be honest she didn't give this to the company. We all had to pick up the slack for her.
While a younger person was put on a development plan and later let go because he did not perform to a proficient level, they let her stay and we just picked up the extra work.