"Derek is ten and Stiles is fourteen the first time they meet. He feels it instantly. It's this hot itch down deep in his stomach-- far enough back to tickle his spine and make his body jerk. He doesn't know what it is, not really, but his instincts let him know it's important, make him pay attention to everything the other boy says, every move he makes.
He's got eyes like warm amber and lips so plush and pink Derek feels flushed just looking at them. His nose is upturned and delicate, his lashes over long-- the beauty marks in clusters across his body an endless fascination. He's got his hair shorn short enough to try and offset the androgyny of his features, but his body's in the limbo state before his presentation. "
tl;dr-- Stiles is Derek's baby sitter and a sexually active omega. Derek wants to be his alpha but hasn't presented yet.