We are born with them, but there can also be injury to the veins from car accidents or gunshot wounds (the former more likely than the latter, at least in the circles I run in) or neck surgery or a few other things that will worsen the underlying condition. Simka has asserted that CCSVI slowly progresses over time, but it's hard to know why that is.
There was a genetic connection found in a paper released in April that showed a link between people with MS and a specific gene abnormality linked to venous malformations. This supports the "born with it" theory, as does the fact that these are often abnormalities like all-sorts-of-wrongness valves that can only result from being formed that way prenatally.
It takes thirty years, on average, before diagnosis because it's chronic slow damage. In Budd Chiari, a liver condition caused by insufficient venous drainage, it takes thirty years before diagnosis but by that time the liver is wrecked and needs a transplant.
Also anything that strengthens the veins (exercise, vitamin D, good diet) is good; anything that weakens them (EBV virus, etc) is bad, but these will only help or hurt to the extent possible dependent on how bad the underlying stenosis is.
At least that's how I understand it! Another significant point is the idea that it requires more than one stenosis to cause this sort of health issues. With just one, the body has alternate routes and can compensate. With 2 or 3 or 4 stenoses or valve abnormalities, including MT syndrome, the body's ability to compensate is overwhelmed.