"Sixteen Going on Seventeen" is a metaphor for fascism.
Essentially, Rolfe is telling Liesl to put aside her own agency and subscribe to his judgement for her own good. He infantilizes her as someone who doesn't know her own interests and paints himself as "someone older and wiser" to whom she should devote herself. In a way, Rolfe is mirroring, between himself and Liesl, Hitler's relationship with the German people.
This explains her fascination with doings things for 'the first time'. She likes repeating milestones. Also, she doesn't mind waiting 'a year or two' because she has all the time in the world.
It doesn't match, of course - it's really
either a studio matte or the Hollywood Hills
- and suitably time-compressed that they haven't traded their Trachten
for Johnson flannels yet, to say nothing about the kids not being any older - but it explains why they're walking in the "wrong" direction. They're headed towards Stowe Village, not Berchtesgaden.
- The valley behind them is recognizable as the Salzburg valley, and the gigantic mountain behind them is readily identifiable, for anyone who's actually been there. Also, the Green Mountains are nowhere near that impressive. And it's not a matte shot.