Where is the explanation of that in the movies? If its in books or something it doesn’t count. And only considering the movies you can infer that they were still clones in episode IV, as Leia remarks, “Aren’t you a little short to be a stormtrooper?’
Apparently not all Clones were killed off, however, they would have been in their 60's or older at the time of Episode 7.
http://starwars.wikia.com/wiki/Stormtrooper
"With the development of the new Imperial war machine, the clone trooper armor was upgraded. Additionally, the new Emperor ordered the extermination of the Jedi Order, the members of which had served as officers in the Grand Army of the Republic during the Clone Wars. Within the new regime, the clone troopers—now known as stormtroopers—were placed under the guidance of Imperial officers and governors instead of Clone commanders or officers. Following the transition, stormtroopers were used to bring any remaining Separatist worlds into line.
As the clones' accelerated aging process began causing their physical skills and abilities to deteriorate, they were replaced by non-clone volunteers and conscripts."
"Stormtroopers were again active approximately thirty years after the Battle of Endor. By then, they were affiliated with a faction known as the First Order, and were equipped with sleeker armor and powerful weaponry."