In 1959, the median age of marriage for women was 20.

Half of the women were married at 20 or earlier.
The 1950s were a time when there was a big problem with premarital sex and teenage pregnancy. Divorce rates had already hit 25%, compared to about 5% before 1910 or 50% today. It wasn’t that idyllic. “Dating” was already well on its way to replacing “courtship.”
But, it sure was a lot better than today.
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Actually, divorce and teenage pregnancy are a big part of the plot of this movie. But, the kids got married in the end.
Here are the original lyrics: