“Our personal histories begin in dependence—first within our mother's womb and then as newborns. Often our life ends in the dependence of old age and the loss of capacities we once had. Personhood is not something we 'have' at some point in this history. Rather, as embodied spirits or inspirited bodies, we are persons throughout the whole of that life. One whom we might baptize, one for whom we might still pray, one for whom the Spirit of Christ may still intercede 'with sighs too deep for words' (Rom. 8:26)—such a one cannot be for us less than a person. Dependence is part of the story of a person's life." [2nd edition, page 6]
I think it is an important point to note that personhood is not something that comes and goes with our state. There may be extreme cases on either end where that may be fruitfully debated, but in my opinion, it is debated in far too many cases.
"Those human beings who permanently lack certain empowering cognitive capacities—as well as all human beings in stages of life where those powers are absent—are simply the weakest and most needy members of our community.”
Their inability to display certain cognitive abilities does not make them less a member of the category “human being.” Very thoughtful article.
You will also want to check out this frightening article at National Review Online.