Newsflash: There were teenagers even in 1938.
Liesl, the oldest of the von Trapp children at 16, is at that weird stage where she's looking forward to being an adult but doesn't quite know how to pull it off. She's flighty and high-spirited, in the throes of a great romance. Or so she thinks. She lets Maria know as soon as she meets her that she does definitely not need a governess.
That said, she's perfectly happy to play young and naïve when she's around her love interest, Rolfe. When he offers to clue her in about love and romance, she coyly admits she's so naïve:
LIESL: Totally unprepared am I
To face a world of men.
Timid and shy and scared am I
Of things beyond my ken.
I need someone older and wiser
Telling me what to do.
You are 17 going on 18.
I'll depend on you…
Apparently womanhood is all about going from having your dad bossing you around to getting a boyfriend to do it.
After a downpour rains out the rest of her evening with Rolfe, she sneaks back into the house through Maria's window. When she sees that Maria will keep her secret, it's the beginning of a great relationship.
You know how it is when your boyfriend joins the Nazi party—he just doesn't have time for you anymore what with all the meetings and propaganda-spreading. When Rolfe shows up to the house with a telegram for the captain, he's curt and cold with Liesl. She turns to Maria for advice and reassurance.
LIESL: Mother, what do you do when you think you love someone? I mean, when you stop loving someone or he stops loving you?
MARIA: Well, you cry a little. Then you wait for the sun to come out. It always does.
LIESL: There are so many things I think I should know but I don't. I really don't.
MARIA: How can you?
LIESL: Sometimes I feel the world is ending.
MARIA: Then you feel it's just beginning?
MARIA: It was that way with me. And for you it will be just as wonderful.
Liesl doesn't have much time to ponder her relationship with Rolfe. She also has more important things to worry about, like escaping and surviving. Any hopes she might have about Rolfe are dashed when he ends up holding the family at gunpoint while they're hiding in the abbey.
Compared to that, aren't your relationships in great shape?