Charmian Carr, who brought the Sound Of Music's Liesl von Trapp to life in the 1965 film, has died at the age of 73.
The actress passed away over the weekend from complications relating to a rare form of dementia, the star's spokesperson said.
Charmian played the teenage Liesl, who falls in love with the Nazi youth Rolfe and sings the now-iconic song I Am Sixteen Going On Seventeen to him in that gazebo scene.
Tributes to Charmian have been flooding in, with Kym Karath – who played Liesl's little sister Gretl von Trapp – tweeting: "It is with infinite sadness that I share the tragic news that the precious & exquisite Charmian Carr , beautiful Liesl , has passed away."
A statement from Rogers & Hammerstein, which wrote the musical, said: "It's always sad when a member of the family passes away - and in the case of the 'family' of the movie The Sound Of Music, it's especially sad when it is the first of the group to go.
"Charmian Carr played the oldest Von Trapp child, and in some ways she maintained that role in real life - guiding, cheering, supporting and generally being there for the rest of her 'sisters' and 'brothers'."
She was a college student when she auditioned for the film, and went on to describe the singalong sessions she frequently attended as a "kind of therapy".
"I tell people that they should consider sing-a-long Sound Of Music like going to a therapist," she said back in 2005.
"It's just a kind of therapy. They can move around. They can dance and talk back to the screen. They can skip their appointment with the shrink that week."
Charmian leaves behind her four siblings, two children and four grandchildren.