Lena Headey was 17 in Waterland when she played the character 'Young Mary'.
That was over 32 years ago in 1992.
This was her first recorded movie acting credit (although may not technically be her first actual acting role).
Today she is 50, and has starred in 61 movies in total, 57 of those since Waterland was released.
In Waterland, I think Lena Headey looks: