I dont really think that age is a problem if they can do the job,
However every longhaul flight I have done with an American carrier (mostly to Japan) has been staffed by large, lazy ladies. Of the elderly persuasion.
Several times one or more of the FAs have been so large that when they are in the aisle there is no way past them. I have oft wondered if this would be a safety hazard.
I have found them to be slow, and tempermental. The whole flight seemed as if they were doing us the favour and we (the customer) were lucky to be there and receive even the bare minumum of service.
The domestic crews seem to be a lot more upbeat, some even seem to enjoy their job!!