Stone Age

The Stone Age was the first period in the prehistory of humankind, preceding the Bronze and Iron Ages and characterised by the creation and use of primitive tools, some of which had a point, and others a sharp edge for cutting. It was also during that period that humankind discovered fire and how to control it. (PROSE: A History of Humankind)

The First Doctor's first trip with Ian Chesterton and Barbara Wright was to Earth's Stone Age. (TV: An Unearthly Child)

A plot by the Prometheans involved using the ancestral memory of the image of the Sun to revert the minds of those on Earth back to the Stone Age. The Eleventh Doctor was able to reverse this by sabotaging the chameleon circuit in his TARDIS, so that the Blue Box image would be implanted into the ancestral memory of Earth's history. (COMIC: Hunters of the Burning Stone)

According to the Silence, they had ruled the Earth since the Stone Age. (TV: Day of the Moon)

The Twelfth Doctor chased, catching her up in the Stone Age. (PROSE: Dr. Twelfth)

Emeron, a planet which was embroiled in a nuclear war by the time that the Second Great Dalek Occupation began, was reduced to a Stone Age level by the Alliance Daleks. (AUDIO: Dalek War: Chapter One)

Other references
Jill Tarrant described the Exxilons as a "Stone Age tribe". (TV: Death to the Daleks)

During the Thousand Year War on Skaro, the Fourth Doctor observed that the discovery of a soldier with a uniform partly made of animal skins with a combat jacket made of "modern synthetic fibre" was like finding the remains of a Stone Age man with a transistor radio. (TV: Genesis of the Daleks)