How long does it take for vitamin b12 deficiency to cause permanent damage?is it possible to reverse?26 years old with 5 years of cognitive problems

Part reversible: Peripheral nerve damage is generally reversible and damage to central nervous system is usually not. Unless you have been a strict vegan, had stomach or small intestinal resection, Crohn disease for years, tape worm a few other things, it is unlikely that your disease is due to B12 deficiency. It would be prudent to consult a neurologist.