Mark Hamill was 22 in Sarah T. - Portrait of a Teenage Alcoholic when he played the character 'Ken Newkirk'.
That was over 49 years ago in 1975.
This was his first recorded movie acting credit (although may not technically be his first actual acting role).
Today he is 72, and has starred in 158 movies in total, 157 of those since Sarah T. - Portrait of a Teenage Alcoholic was released.
In Sarah T. - Portrait of a Teenage Alcoholic, I think Mark Hamill looks: