How do you determine whether a coconut is young or not by knocking on it? I've seen some people knock on a coconut to determine if it is young or not. Sometimes they knock with their knuckles, or with a machete. How do they do that?

A young coconut has lesser meat, but softer and almost-translucent. The coconut water also tastes better.

Edit: Take note that the key word here is KNOCK. I generally don't see them shaking the coconut.
Shake it gently and listen for and feel the slosh. As coconuts age the liquid level decline to zero

An unripe coconut is nearly full and can hardly slosh. A good slosh sound is a generally ripe coconut (about 150 to 300 ml's of liquid). As the liquid get less and less the flesh get softer and tastier to eat raw

There are many different types of coconuts, this may not apply to them all?