Vitamin B12 Deficiency by A Hunt, D Harrington, S Robinson, BMJ 2014 says:

"In the United Kingdom and United States the prevalence of vitamin B12 deficiency is around 6% in people aged less than 60 years, and closer to 20% in those aged more than 60 years. Across Latin America approximately 40% of children and adults have clinical or subclinical deficiency. The prevalence of deficiency is much higher in African and Asian countries — for example, 70% in Kenyan schoolchildren, 80% in Indian preschool children, and 70% in Indian adults. In vegan and vegetarian groups the rates vary — in the United Kingdom, 11% of vegans are deficient in vitamin B12 and in Ethiopia 62% of vegetarian pregnant women are deficient."

Vitamin B12: All Your Questions Answered (ForksOverKnives) explains:

Who is at risk for B12 deficiency?

Unfortunately, many people appear to be at risk, as vitamin B12 deficiencies are fairly common in the general population. One study found that 40 percent of children and adults in Latin America had deficient or marginal status. Another study found that 20 percent of those over the age of 60 in the U.S. had a marginal status.

Another review found that 40 percent of patients in the U.S. had unexplained low vitamin B12 levels. Researchers attributed this shortfall to “food cobalamin malabsorption,” meaning the vitamin B12 that is naturally present in foods is simply not absorbed. Luckily, research has shown that these people are still able to absorb the crystalline form found in supplements and B12-fortified foods.