The Tripods series by John Christopher

Sam Youd (16 April 1922 – 3 February 2012), known professionally as Christopher Samuel Youd, was a British writer, best known for science fiction under the pseudonym John Christopher, including the novels The Death of Grass, The Possessors, and the young-adult novel series The Tripods. He won the Guardian Prize in 1971 and the Deutscher Jugendliteraturpreis in 1976.

The story of The Tripods is a variation on post-apocalyptic literature, wherein humanity has been enslaved by "Tripods": gigantic three-legged walking machines, piloted by unseen alien entities (later identified as "Masters"). Human society is largely pastoral, with few habitations larger than villages, and what little industry exists is conducted under the watchful presence of the Tripods. Lifestyle is reminiscent of the Middle Ages, but small artefacts from the Modern Age are still used, such as watches.

Humans are controlled from the age of 14 by implants called "Caps", which suppress curiosity and creativity. Some people, whose minds are broken by the Caps, become vagrants. According to The City of Gold and Lead, Masters begin to believe that humans should be capped at an earlier age "because some humans, in the year or two before they are Capped, become rebellious and act against the masters"; but this cannot be done because Capping must wait until the braincase has stopped growing.

The books in suggested reading order
1. When the Tripods Came
2. The White Mountains
3. The City of Gold and Lead
4. The Pool of Fire