The study was published in the Annals of Neurology and it comes from a collaborative study between Örebro University and Karolinska Institutet, which showed concussion in adolescence increased the risk of MS in later life by 22 per cent for one concussion, and teenagers who experienced two or more concussions were at more than a doubled risk of MS – 133 per cent.

But not all teenagers run the same risk:
"MS is caused by a combination of genetic susceptibility and environmental exposures. Most of the young people who experience a head trauma should not worry as they will not carry the necessary genes and other risks that will result in MS in later life," says Scott Montgomery

https://medicalxpress.com
/news/2017-09-concussion-teenagers-multiple-sclerosis-life.html

Concussion in teenagers increases the risk of multiple sclerosis (MS) in later life. However, there is no association with MS for concussion in younger children. This was reported by a new study, led by Scott Montgomery, Professor at Örebro University in Sweden.