system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do old people sometimes have shaky voices?

What physically causes it?  Thank you.
gpt
Vocal cords vibrate - that’s part of their function. And vibration is a really great way to cause wear and tear on any material. Especially over long periods of time, vibration can wear down vocal cords and the muscles that move them so that motor control over them degrades. 

Certain medical conditions associated with old age are also associated with a loss of muscle control which can affect the vocal cords. And being older also increases the risk of regularly inhaling things like cigarette smoke which can also damage tissue along the throat.

Another cause is the respiratory system. Older people are at higher risk for respiratory conditions which make the power of their lungs reduced. At that point, they may have trouble sustaining airflow for a stable voice.