In Thailand we eat young coconut much more than old coconut. We use it to make desserts, we make drinks from the flesh and use it for many recipes. We also use it like this - we drink the juice from inside.
If you have only drank the water from old brown coconut, then you are in for a very very pleasant surprise, a young coconut is full of sweet delicious juice. To open, use a machete or Thai cleaver and make a few cuts around the top to open it, as in the photograph.
Be careful, hold the coconut by the base and swing the cleaver away from you, towards the top. Then if it slips you don't hurt yourself.