For a teen, the best way is to buy a good mining rig and start mining. It will take a few months to pay for itself, however you will learn far more about Bitcoin by mining than by any other way.
If you buy Bitcoin they will just sit there, then you will spend them. Game over. If you mine, it will be for life.
I'm sure mommy wouldn't want to foot the bill after giving her teen that much money. The money and time spent mining and reselling the obsolete equipment afterwards is just not worth it.