He said she was young (spoilers)
At some point the baron tells Lockhart that he shouldn't be fooled by Hannah, as she just looks like a woman, but she is actually a little girl inside.
Seems to me the context was the exact opposite: she looks like a girl, but she's 200 years old.
And while we're here: why was she considered underdeveloped when she had all the time in the world to become exceptionally mature and wise (a la Interview with the Vampire)? Don't tell me she just went around the fountain feeding fish every day for a few centuries until she realized this guy Lockhart (supposedly ~35, but looked 19) was attractive.