Forever Young

• WWizards of the Coast2019-10-04
  Official Ruling
  You can cast Forever Young with no targets if you just want to draw a card.
• WWizards of the Coast2019-10-04
  Official Ruling
  If you choose any target creature cards and they are all illegal targets by the time Forever Young tries to resolve, the spell won't resolve. You won't draw a card.
• WWizards of the Coast2019-10-04
  Official Ruling
  You choose the order that the creature cards are put on top of your library. You don't have to reveal this order to other players. Whichever ends up on top will be the card you draw.

Rulings via Scryfall