You can prevent it
If you're a pure vegan- you are at risk to develop vit B12 deficiency.
It'll take a few years to develop vit B12 deficiency anemia
from being a pure vegan.You should take vit B12 supplement to make sure you are not deficient.If you still eat egg & milk- you can have some vit B12 from them-however may not be sufficient. Get supplement. Use B12 fortified food and look for cyanocobalamine on the label.