Sylvester Stallone was 21 in The Square Root when he played an uncredited character.
That was over 55 years ago in 1969.
This was his first recorded movie acting credit (although may not technically be his first actual acting role).
Today he is 77, and has starred in 135 movies in total, 130 of those since The Square Root was released.
In The Square Root, I think Sylvester Stallone looks: