The older you are, the more likely you are to be on medication aimed at controlling / reducing acid production in the stomach.
Now, there are a number of issues with the long term use of these medications, but until now, low Vit B12 levels were not considered one of them. But see this article.
And another study has highlighted how low Vit B12 levels increase the risk of bone - especially spinal bone - fractures in the elderly.
Worth bearing in mind.