A soldier boy when he proposedShe said no; his face just frozeYou know she loved himBut was so youngCan't marry no oneCan't marry no one
And here's the first verse of "I'm So Young":
I have a girlfriend; she says I'm her only oneWe wanna get married, but we're so youngSo youngCan't marry no one
Both end with "So young / Can't marry no one." Furthermore, as I've noted before, in both, "young" is sung with a melisma, providing a sense of degree (for "so").
According to the liner notes of the respective albums, "A Casual Look" was written by Ed Wells, and "I'm So Young" by William Tyus.