How long would a baby in a crib survive without parents tending, without mother's milk?
How long would a child alone?
For that matter, how long would a sole adult survive if left naked in the woods even at this time of the year?
Just because a person at whatever age/stage wouldn't last long on their own does not make them not a person, it makes them a mammal, part of a social species, a heavily dependent one, one that in fact undergoes the longest child rearing period of any animal to have ever existed.
People's selfish behavior and the lack of responsibility that goes with it defines their stunted morals, not whether another person qualifies as a person.