Mike wrote --

 MP> Was she actually a young girl, or a character who was pretending to be a
 MP> young girl?  I was paging through the movies that are coming on TCM the
 MP> next couple of weeks and saw one that sounded similar, but was about a
 MP> character who was not really young but was pretending to be.

   Sounds like The Major and the Minor, with Cary Grant and Ginger Rogers,
where Rogers is short of funds and pretends to 12 years old to get half
fare on a train.
   But I don't think that's the same film Ed is thinking of.
   Joe