Are Imperial stormtroopers all clones?
They were initially all clones (of Jango Fett), but to them other clones and enlisted humans - soon considered largely inferior - were later added. See more in the Wookieepedia article.
The article sources an extended universe novel. But I'm inclined to agree with this answer simply because the original clones would have aged by the time of episode IV. Had they been engineered to age slowly, the Kaminoans would have mentioned it to Obi-Wan. But then again, Imperial scientists could have used the DNA of the Jango Fett clones to make more clones of the same type.
According to the article, as of 0 BBY about a third of Stormtroopers were Jango Fett clones.
Palpatine also shut down the clone production after his victory over the Jedi, after that it would have been a process of enlistment.