Gillian Anderson was 16 in Three at Once when she played the character 'Woman 1'.
That was over 38 years ago in 1986.
This was her first recorded movie acting credit (although may not technically be her first actual acting role).
Today she is 55, and has starred in 45 movies in total, 44 of those since Three at Once was released.
In Three at Once, I think Gillian Anderson looks: