Wait I think I need to clarify, your character does not need to be between the ages of 13-19. That timeframe is only for when your character got their powers, they can be older than 19.
The bio should be a quick explanation of their normal life and their life after their turning. As for the Post Example it doesn't have to be long just try not to make it a 5-8 sentence thing. Like two paragraphs or more would be fine.