Earlier quoted context omitted.
"megadosing" doesn't sound like its required for any reason unless you're malnourished, in which case your doctor would tell you how much to take. Adjusting your diet to not require "megadosing" due to malnourishment sounds like a better plan for long term health.
In addition, studies suggest that high levels of B6 and B12 in the blood are correlated with higher risk of certain cancers, such as prostate and lung cancer. In my opinion, having mild nutrient deficiency is not cause for alarm if you're a fully grown adult, and in fact, studies suggest that malnutrition might confer a longer lifespan.
There are some data suggesting that inadequate supply of this or that nutrient reduces certain aging mechanisms and leads to longer lifespans in a laboratory setting. However, outside of a laboratory setting these nutrient deficiencies can affect the body's ability to fight infections and heal injuries, which on average cancels out any benefits.
More generally, trying to increase lifespan by adjusting the diet seems to me like trying to fix a car by playing with the buttons and knobs on the dashboard. You might obtain a small improvement, but you're not really going to get anywhere unless you open the hood.