This is an old thread, but I have to comment after seeing several filmed versions of the classic "Little Women" and being a lifelong fan and frequent re-reader of the original book, I feel that Katherine Hepburn was born to play Jo!
I don't think she looked too old, because, as in the book, she was quite mature and independent and for example, took short trips to try to get her writing placed with publishers, which would have been very unusual for a girl of her time, as well as later going off to be a governess and try to write in NYC. Also, Jo had a relatively hard life, and they didn't wear makeup and skin renewers in those days(of the book), as well as being a tomboy she wasn't interested in trying to look feminine most of the time! Remember, girls of 16 and even younger during the Civil War were quite often already wives and mothers.
They were not carefree adolescents as many 16-year-olds are in our times, concerned solely with school, school activities and friends, but were raised to know how to run a household and cook, clean, wash clothes, etc. I'm not saying that many girls nowadays are not mature at 16 and also know how to do all those things, but our society's attitudes toward what a teenager is have definitely changed.
Ms. Hepburn embodied both physically and mentally the tomboyish and independent spirit of Jo from the book perfectly, much better than June Allyson or Wynona Ryder did later. Ryder is my second favorite Jo, though. Katherine Hepburn's "Hepburn-ess" worked perfectly for the characterization of Jo, as she was the same type of tomboyish and independent woman. With Ms. Hepburn's performance, as well as the overall quality of the production, this is definitely the superior version of the classic Alcott work.