When were the Clone Troopers replaced with regular humans?

Question

It is clear in the original trilogy (Episodes IV-VI) that the Storm Troopers are regular humans. Mostly because of the funny scene, where a tall Trooper hits his head on a low door while his colleagues pass through with ease. So they are not clones but individual humans with different body types. At the end of the prequels (Episodes I-III) it seems the whole military is made of the clones and the droids we've seen attacking Naboo. So when did the Empire stop using clones, e.g. replace fallen clones with real humans again? Side question: Were there units where clones and humans fought together?

There is a related question: Why did the Empire discontinue the clone trooper program? However, the other question asks why, whereas this one asks when.

Answer 1

Clone troopers were almost entirely replaced by human volunteers within 5 years after the end of the Clone Wars.

The canon novel Tarkin shows that most stormtroopers were regular humans and that there were only a few aging clones left (emphasis added):

When the stormtrooper operating the equipment accidentally allowed [Vader's meditation] sphere to bang against the edge of the cargo hold’s retracted hatch, Vader stamped forward with his gloved hands clenched.

“I warned you to be careful!” he shouted up at the trooper.

“My apologies, Lord Vader. Wind shear from—”

“Excuses won’t suffice, Sergeant Crest,” Vader cut him off. “Perhaps you are aging too quickly to remain on active duty.”

Tarkin couldn’t make sense of the remark until he realized that Crest’s was a face he had seen countless times during the war—the face of an original Kamino clone trooper. The bare-headed others comprising Vader’s squad were human regulars who had enlisted after the war.

_{Tarkin, p. 94}

Since the human regulars were said to have "enlisted after the war", it sounds like production of the clones ended with the Clone Wars and the Empire began phasing them out immediately.

The five year upper bound is provided by the fact that the novel takes place five years after the end of the Clone Wars and the creation of the Empire:

A long time ago in a galaxy far, far away...

Five standard years have passed since Darth Sidious proclaimed himself galactic Emperor...

_{Opening crawl for Tarkin, p. 4 (a copy of the opening crawl is also available on Facebook)}

Answer 2

In Star Wars Battlefront II, it gives some background on this (Legends). Originally, all clone troopers were made on Kamino and they were Fett-clones with enhanced aging. Between episode three and four, the Kaminos had a rebellion, using an army of 'corrupted' Fett-clones. After that was suppressed it was decided to use more than just Fett-clones. But clones nonetheless.

The tall trooper in question could have been a clone of just a taller individual. So the premise "a tall Trooper hits his head on a low door while his colleagues pass through with ease. So they are not clones" is not a solid premise.

In Episode 7, we find out that the First Order uses regular humans but they were kidnapped as children and brainwashed.

I'm not sure when regularly humans got introduced but eventually they did.

Answer 3

Clones were subjected to age acceleration, after 20 years, new non-clone recruits were put into stormtrooper duty. You get what you pay for and the Empire being broke, after projects like the Death Star and Tie Defender failures, their new recruits couldn't shoot the broad side of a barn. In A New Hope when Luke and Ben come across the blasted carcasses of the Jawas and Ben points out that only Imperial Stormtroopers are so precise he is thinking 20 years out of date.