Hm. While I kind of like the idea of mages jonesing for whole elf milk for quick, cheap powerboosts, like gaining a brief burst of raw mana, there's other ways. Mana Milk could work, but while all elves are magical they aren't all magic users. So maybe instead of the magic why not focus on the life aspect? Elves wean their kids on liquid lifeforce since they're so long lived. Humans and others find out it's a fast way to extend life. Kind of fits better with the idea that the more you use it the more you end up looking like an elf since as you increase your lifespan that's pretty much what you're becoming anyway.