How do you determine whether a coconut is young or not by knocking on it?
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?