Fermented foods such as kimchi and sauerkraut can supply B12, as can seaweed and some other foods.
So it is possible.
So it is possible.
All experts recommend vitamin B12 supplements for vegans, vegetarians and older people.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4042564/
https://www.ncbi.nlm.nih.gov/pubmed/17959839
https://www.b12-vitamin.com/algae/
From https://en.wikipedia.org/wiki/Vitamin_B12_deficiency:
> Vitamin B12 deficiency in the US and the UK is estimated to occur in about 6 percent of those under the age of 60, and 20 percent of those over the age of 60.[1] In Latin America rates are estimated at 40 percent and they may be as high as 80 per cent in parts of Africa and Asia.[1]