Here's an article about a recent French study about the effects of obesity on cognition over time...
http://tinyurl.com/y6l55q
It appears that this is even more evidence that there is a strong correlation between obesity and the loss of mental capacity as we age.
It should also be kept in mind, I think, that one of the factors that keeps us mentally agile as we grow older is physical activity and exercise. Obviously, someone who is obese is less likely to exercise, so I don't know to what extent one plays into the other, although I expect it creates a sort of feedforward cycle.
My best advice? Don't weigh anymore then you have to to feel wonderfully healthy and certainly no more than allows you to be joyfully active! Speaking as someone who has lost five stone in less than a year and is still noticing the pounds drift away, I can tell you that it's definitely something you can do even more easily than you might have thought!
Be Well,
Michael Perez