==Nonsense!==  
 This sort of stuff offends me.  Please do not refer to a woman of 20 as a "young (American) girl".  She was not a "young girl".  It is horribly paternalistic and utterly chauvinist to refer to her as such, and even more so to make front page DYK with it!   

 She was with another woman. One of them was about twenty, the other one was about thirty.  Bending the facts in order to use the word "first"  as in "the first young girl"is really inappropriate.  Both women were of marriageable age, and indeed many women have several  children before they reach the age at which Harriet went to China.  

 A "young" girl might be one of four to six, i.e. no longer a baby, definitely a girl, but a "young" one.    

 Call Harriet a "young woman" if you must insert an adjective.  Her aunt could also be called also a "young woman" unless she had children a was referred to as a "matron"