Did you read the link? Sarcopenia begins in the 4th decade...

Inactivity, disease processes, advance it.

When nerves malfunction, the signals to and from them change.
For example, proprioception nerves tell the brain what the body is doing. If the brain stops sending signals, the muscles stop
working.

There are autoimmune connections to muscle as well. For those with autoimmune antibodies, the neuromuscular junction may be affected.

There have even been studies to show quality protein can stave off sarcopenia. Studies on Whey protein show older patients who use this amino acid supplement show less muscle changes/loss with age.

Muscle atrophy beyond age related sarcopenic loss should be brought to the attention of your doctor. It may change your diagnosis.

Perhaps you should get some genetic testing? I don't recall if you have done that before?