How old is Marguerite?
Like you said, her age hasn't been officially revealed yet. Judging from her physique and face, she could be anywhere from 16 to 24 years old, but until it is brought up in-story or Oda clarifies it in an SBS, her age will remain unknown.

EDIT: The Vivre Card Databook Vol. 4 finally revealed Marguerite's age; she is 16 pre-timeskip, and 18 post-timeskip.