I like to look these movies up on IMDB after I view them, so of course I did for this one. It perplexed me that Beth was made younger than Amy, and my original theory was that they wanted a younger character to die to make it more “heartwrenching.” After looking at the trivia for the movie however, the rest of the class was right (or Dr. Campbell, I can’t remember who suggested this) and Beth was made younger because of the availability of the actress.
I became interested in why this occured, so I found this nifty page on Wikipedia about the studio system.
It is interesting to note that Margaret O’Brien, who played Beth, was only really known for being a child actress. She wasn’t even named Margaret, her first name was Angela. She took the name Margaret after her first major role, as was expected for a child star. Talk about the movies controlling your life.