Margo Harshman was 9 in The Elf Who Didn't Believe when she played an uncredited character.
That was over 27 years ago in 1997.
This was her first recorded movie acting credit (although may not technically be her first actual acting role).
Today she is 38, and has starred in 16 movies in total, 15 of those since The Elf Who Didn't Believe was released.
In The Elf Who Didn't Believe, I think Margo Harshman looks: