This has nothing to do with “ageism”, which is an statistical idea applied individually.
While I disagree with that statement, especially in this context, I'm glad to see that you understand the difference between discussing statistics about a demographic population (identified by observation of past events) and inappropriately applying those statistics to an individual.
When you said
The elderly need to SIT DOWN. It’s not their planet anymore, and they need to get over it.
You were insisting on specific future actions ("SIT DOWN" and "get over it"). Actions are taken by individuals. Age is a characteristic that individuals do not have control of. It is not a decision, and we don't cast aspersions on people for things they do not have control of.