Maggie Smith was 20 in Child in the House when she played the character 'Party Guest (uncredited)'.
That was over 68 years ago in 1956.
This was her first recorded movie acting credit (although may not technically be her first actual acting role).
Today she is 89, and has starred in 90 movies in total, 88 of those since Child in the House was released.
In Child in the House, I think Maggie Smith looks: