No children should be playing contact sport before the age of 12, say some of the world's leading experts on head injuries in sport.

The group of Boston scientists told the Herald that the younger you started sports like rugby and American football, the greater your chances of suffering degenerative brain diseases.

Dr Bob Cantu, the "Godfather of Concussion", has also discouraged football clubs from allowing heading the ball in under-12s.

In a series of exclusive interviews, the scientists at Boston University's Alzheimer's Disease Center team showed experiments which found mice subjected to the same forces as tackle football suffered