Speaking as a vegan of over 10 years, who is mentally sharper and healthier than I was a decade ago, I also don't agree with the "steep decline in health, both physical and mental". I do agree that certain nutrients, such as B12, need to be supplemented, but the Institute of Medicine recommends that everyone over the age of 50 supplement B12 whether vegan or not.
Not actually true. For centuries, the traditional Chinese diet did not include dairy and minimised meat consumption too. The oft-trotted-out trope of calcium in dairy is misleading (http://www.hsph.harvard.edu/nutritionsource/calcium-and-milk...) and even relatively small amounts of meat and dairy increase the possibility of prostate cancer (http://www.ncbi.nlm.nih.gov/pubmed/18398033)
Speaking as a vegan of over 10 years, who is mentally sharper and healthier than I was a decade ago, I also don't agree with the "steep decline in health, both physical and mental". I do agree that certain nutrients, such as B12, need to be supplemented, but the Institute of Medicine recommends that everyone over the age of 50 supplement B12 whether vegan or not.