Yeah, one of the main reasons why older adults don't gain muscle at the same rate as when they were younger is that the reservoir of muscle progenitor cells (satellite cells) depletes and that more of cells within that reservoir become senescent.

Fortunately, while this process slows muscle growth and muscle maintenance, it never completely abolishes our ability to escape from sarcopenic frailty. That's why even in 80- or 90-year old people a proper training regimen can have an anabolic response.

We have to live with the "immeasurables" for the time being. But we have one consolation: since exercise training and dietary choices have a systemic effect, it is reasonable to suspect that the functional benchmarks that we can measure (arterial function, muscle function etc.) are representative for improvements of the immeasurable functions, too. Indirect evidence for that hypothesis is aplenty.