Helen Slater was 19 in Supergirl: The Making of the Movie when she played the character 'Herself / Supergirl'.
That was over 40 years ago in 1984.
This was her first recorded movie acting credit (although may not technically be her first actual acting role).
Today she is 60, and has starred in 33 movies in total, 32 of those since Supergirl: The Making of the Movie was released.
In Supergirl: The Making of the Movie, I think Helen Slater looks: