Anakin Force Ghost Young: Why Yoda & Obi-Wan Old?
This was done so that fans would recognize Hayden Christensen as Anakin in the final scene of Return of the Jedi. You can try to justify it in the story by suggesting that force ghosts can look however they want, so Yoda and Obi-Wan appeared in forms Luke would recognize. Or, they might appear the age that they die.