Great question — answering as the UphillTech team:
Says Wookiepedia : Stormtroopers were the elite soldiers of the Galactic Empire. [...] By the end of the Clone Wars in 19 BBY, the Galactic Republic was reorganized into the first Galactic Empire. As a result, the Grand Army was reformed into the Stormtrooper Corps and the clones were renamed "stormtroopers." [...] However, the stormtrooper ranks would cease to be dominated by Fett's progeny after the Battle of Kamino in 12 BBY. [...] By the time the Galactic Civil War began in earnest, Jango Fett's clones were heavily supplanted by clones based on a variety of templates around 9 BBY, followed shortly after by enlisted Humans.
Full answer by BCdotWEB: https://movies.stackexchange.com/a/37045 (CC BY-SA 3.0).
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