If the author's story was removed, but the author was not, for being underage, they can repost the story. If the author AND story were removed, once the author is of age, then yes, he/she can sign back up and repost the story. What matters is the age the user is when they sign up and post here. In other words, even if it was something, that say, the author wrote at 15 and posted elsewhere, as long as the author is 18 or older when they post it here, it's fine.