Tina Andrews who played Mary was 23 years old in real life!

I remember watching this movie as a 8 or 9 year old and remember seeing Tina Andrews for the first time in my life as Mary. She of course is the girl who Conrack get's to join the school. She was 13 in the movie and I thought she had to be no more than 14 in real life, but than years later I found out she was 23 years old in real life! I was surprised because she was so belieavable as a 13 year old girl. Julie Harris played a 12 year old when she was 22 in A Member Of Wedding and she was not a believeable 12 year old. She was nominated for an Oscar of course(which was a debatable nomination) and I'm not saying Tina should have been nominated too, but I think her performance is underrated and was a very good one. Anyone else shocked when they found out her real age?