They can be overpowered by the human revealing the elf's true age. Elves tend to be conservative, and hate iron, steel, religion, and industrialization - blaming these for the separation between the elves and humans. They despise humans who follow new ways - even to the point of tabooing using salts and spice on their food. Elves supposedly have hollow backs and are never quite corporeal in our world. Elf-women have long drooping breasts like do many animals and work the fields while suckling numerous children at the same time.
Some elves have human appearances with exception of animal feet, ears, skin, or teeth.
They despise materialism. I could go on listing forever.