B12 issues can arise at any age and result in crippling neurological disorders

 You may find yourself among those who must ask whether their difficulties are Alzheimer's or vitamin B12 deficiency. Whether you inherited a disposition toward difficulties with B12 metabolism or it comes to you down some other path, you and your physician may be slow to recognize it.

The blood tests that are part of a typical yearly physical in this country do not include routine examination of vitamin B12/folate, regardless of your age, because it is comparatively rare.  According to the National Institutes of Health,  National Institutes of Health,  analysis of NHANES data from 2007–2018, approximately 3.6% of all adults age 19 and older have vitamin B12 deficiency," though by their definitions, I was one of them in 2019. Very unhappily. And thanks to a discerning and well-informed physician, I escaped the horrors and apparent permanent damage others suffer.

A 2021 review published in the International Journal Of Molecular Sciences gives good summary attention to the neurological symptoms, which typically include a sharp decline in cognitive ability, and cites multi-year studies which show that recovery is in some regards slow in coming. 

Anyone can suffer the reversible dementia, psychotic symptoms and seizures involved. The sixty-one-year-old patient referred to in this article  was said to have "had a remarkable recovery" after vitamin replacement therapy. Limited recovery, however. Delayed diagnosis apparently resulted in "irreversible axonal damage." 

Though the price of delayed diagnosis can be very high, this 2024 study found no clinical consensus regarding diagnosis, treatment and long-term management of B12 deficiency in adults. My endangered genetic kin are very much on their own.