I consider a classic something made at least 40 years ago… I know most of the ages of directors, but imagine if “One Flew Over The Cuckoos Nest” was directed by an 18-year old…
— Or, a young writer if you can’t think of any directors.
Citizen Kane. Welles was 26.
Steven Spielberg was 29 when he directed Jaws. Maybe 28 – it was released when he was 29.
Battleship Potemkin by Sergei Eisenstein was released when he was 27.
2 more recent classics, Quentin Tarantino was 28 in 1992 when Reservoir Dogs was released. And Steven Soderbergh was 26, the youngest solo director to win the Palme d’Or at the Cannes Film Festival, for Sex, Lies and Videotape in 1989.
The recently deceased John Singleton was 22 when he was nominated for directing Boyz N the Hood.
Charlie Chaplin started directing short subjects at age 25 and started putting out classics two years later.
A bit too recent for your criteria, but Kevin Smith directed Clerks at age 24.
George Romero was 28 when he made Night of the Living Dead
John Landis was 27 when he made Animal House
Clerks, Kevin Smith, 24 years old