It looks a lot like Valerian too. They shoot up quick like that as well. Leaf is a bit more serrate than the elder, but that can vary quite a bit on your seed source. I've had Valerian 12 ft tall in my garden.
My elderberry is the woody stem type but I've seen pictures of green stem types like that. Must be great soil there!
But nothing is that big just coming from seed, so I'm guessing the patch has a history with said thug.
