Christian Slater was 11 in Sherlock Holmes: The Strange Case of Alice Faulkner when he played the character 'uncredited'.
That was over 43 years ago in 1981.
This was his first recorded movie acting credit (although may not technically be his first actual acting role).
Today he is 54, and has starred in 108 movies in total, 101 of those since Sherlock Holmes: The Strange Case of Alice Faulkner was released.
In Sherlock Holmes: The Strange Case of Alice Faulkner, I think Christian Slater looks: