A quarter of the roughly 500 gang criminals in our review were born in the 21st century and the people are usually registered in the areas where they commit crimes.
The youngest gang members in the survey are 16 years old and are often related to other people in the networks.
They are primarily convicted of drug and weapons offences.
An example is a 16-year-old, who is a friend and crony of the leader of the Sigma network in Upplands Väsby.
He was sentenced to six months in closed youth care last year, after possessing weapons used in a shooting at a rival gang in the same area.
In a series of special interviews in the surrounding areas, local police officers explain how conflicts between areas flare up - and escalate.
- In Bredäng, the gang conflict began in 2013 with a suspected theft from their own warehouse.
It has claimed many human lives, says David Sund, group leader in the Skärholmen local police area.
From running boys to killers – the role of children in gangs
The majority of the people in the review are convicted of various types of drug and weapons crimes and murder, attempted murder and assault.
But the police see how increasingly serious crimes are committed by children.
- Unfortunately, we have children who are lining up to belong to a criminal network.
Previously, they were more runaway boys - now they carry out murders, says Anna Rise, head of the intelligence unit in the Stockholm police region.
The police are starting a new survey
The 500 people included in our survey make up a good third of all gang criminals in Stockholm County, according to the police's previous calculations where the core but not the tail is included.
Now the police are carrying out a new survey and think partly differently.
- I don't think we got more individuals - on the other hand, we need to review which parts belong to a network and the role of young people, says Anna Rise.
Stockholm's gang criminals are on average 21 years old - hear about the police's new mapping and why violence is increasing right now - in the video.