Hermes was, I think, four days old, when he stole Apollo's cattle. He was the god of cattle driving and thievery and trickery and cunning, amongst other things of course, so it makes sense that one of his first acts would be something like that. The myth goes that, as an infant still wrapped in swaddling clothes, Hermes escaped from his crib and flew to Pieria where his brother Apollo kept his cattle. Hermes swooped down and led the cattle away. He put on boots to disguise his tracks and led them to a grotto in Pylos. I think one part of the myth went that Hermes came upon a sheep herder and told him that, if anyone (namely Apollo) came by asking about the herd, that he should lie and say he hadn't seen them. To test whether or not the man would actually do what he promised, Hermes doubled back and, disguising himself, asked the man if he'd seen a boy leading a herd of cows. The man replied that he had and so Hermes either cursed him or killed him. Then when night came and went Hermes ran back home and jumped back into his bed pretending to be a wee babe, but his mother knew what he had done (as mothers tend to be able to tell whenever their kids have done something wrong) and she told him he was a devilish rogue and he'd get in trouble if he didn't change his ways. Blah blah. I'm pretty sure also that during that little excursion is when he invented the lyre and the fire stick.
Homer and Ovid give different versions of this story. It's possible I'm mixing the two.