I think Dr. Zamboni has suggested that CCSVI may be congenital. Other causes may be due environmental changes or possibly trauma. I've read that he did do a study back in the late 80's on the neck veins of teenagers/children, and 90% of them had a MS diagnosis by their twenties. The name of the paper is "P. Zamboni, A. Cossu, L. Carpanese, G. Simonetti, G. Massarelli, A. Liboni: "The so called primary venous aneurysms", Phlebology 1990, 5:45-50 "
I haven't been able to track it down as of yet, it is only coming up as a reference.
As with anything (food, alcohol, drugs, smoking etc) everyones body will cope differently when placed under stress. Some people can smoke a packet or more a day all their lives and not get cancer, others might not smoke as much, yet get cancer in their twenties. We just don't know when each individuals body will say enough is enough and start to be affected.