The tests here for methylation DNA errors are now patient driven. Called the MTHFR DNA test.
Some places will do them without a doctor. They range in price from just over 100 American dollars to about 500 for a complete evaluation of liver enzymes.

Overtime as people age, yes, there are impacts on the nervous system and brain. Pernicious anemia can result. High homocysteine levels will cause damage to the cardiovascular system, and lead to heart attacks/stroke and peripheral artery disease.

I know of one preteen who was diagnosed early. And she had
also gluten intolerance which can occur with malabsorption of nutrients.

You can try plain folic acid... just take some B12 with it.
See what happens.

I'd encourage you to look at DHA as well. Vit D has been shown to cause depression if levels are very low. So folate is not the only player in this problem.

Vit D testing is being aggressively done in USA now. If you Google it there is a wealth of new information to be had on it as well. I don't know what is happening where you are however.

We have a Vitamin forum here with many many posts on Vit D and B12 so take a look!
http://neurotalk.psychcentral.com/forum49.html