When muscles go in adults it's due to individual muscle cells getting bigger and not due to increase in muscle cells. It's generally believed that no new muscle cells get produced in human adults. Sometimes muscle cells die. I would expect that as humans age therefore the count of muscle cells will go down. There are frequently claims that there's a limited number of hallmarks of aging that if repaired would stop aging. Neither SENS list nor the 2013 paper seem to include muscle cell count as one mark of aging. Is there a good reason for this?
Actually, they all do include it, but is is subsumed under stem cell aging, loss of cells and reduced regenerative capacity. Also to clarify what I would consider a misunderstanding. Not everything has to fit. There are probably infintely many causes of aging or at least quite a lot. Most of these fall into the rough categories or "hallmarks" we have come up with like reduced stem cell functioning or damage to biomolecules. Many of these causes are not relevant to immediate life extension which is why they can be ignored for now. Other categories or "hallmarks" will be discovered as we go along.
Having said that, dysfunction of the neurmuscular junction is probably the most important type of muscle aging, much more so than cell loss, and, being so complex, I do not think the hallmarks do it much justice. Many of the hallmarks are so vague as to be almost useless anyway.