Crothian said:Not that this is ever talked about in the game, but it seems to imply that creatuires are of their own alignment from birth on.
I disagree. The PHB rules are pretty clear - in general, creatures that cannot make moral decisions are Neutral. I'm pretty sure infants fit in that category.
At some point in their development, children of sentient species gain enough life-context to understand their choices (at least to some extent). At that point, and not before, they may become aligned. Exactly when that point comes depends upon your take on psychology, and humanoid developmental speeds.