Older people are around just as long as young people, and learn just as much (yes, in some cases more, though in my experience as a manager that is not the usual outcome, it usually falls off slightly). The difference is the baseline: a person 1 year in should at least double their experience within the first year of working for you, whereas a person with 10 years experience will increase it by around 10%. Any way you cut it, measuring the current state matters a lot more for the more experienced person than the young one (current skill is an estimator of how fast the more experienced person has been learning, whereas for a new grad it's on you to estimate that based on other factors, and current skill doesn't matter much because it will change so fast).
I strongly disagree. Why would I not be around as long as a younger person? Most young people last about 3-5 years in a job as far as I can tell. As I get older, I'm jumping around less. And I'd say that my skills are growing faster than I was when I was younger. I don't waste time on a lot of fluff that I would have when I was younger. I focus on areas of growth that are more likely to have an impact, not only on myself but on the rest of my team. I see the same patterns in other older engineers I work with.