One thing I would point out is that, in terms of what's "canonical", this is perhaps partially dependent on which edition of D&D you're using. Monster Manual entries in some (perhaps even most, nowadays) editions frequently state that a creature's alignment is "Always [x]" or "Usually [y]", and may also contain things like "Any good" etc.
For those creatures labelled "Always chaotic evil", it seems to me that this "Always" is interpretable as being an intrinsic characteristic of the creature, rather than a product of its upbringing, whereas "Usually" denotes more flexibility, perhaps implying that the creatures in question are more prone to making a personal choice early in life.
This in turn is not to say that an intelligent creature, born to chaotic evil, cannot decide later in life to turn from that path and become good. A strictly defined alignment (anything other than "neutral") implies an understanding of the moral consequences of one's actions, and, beyond a certain point, denotes a choice to behave in a moral or immoral way. Is the mountain lion who drags off a small child to feed itself inherently chaotic evil? No, it is an amoral predator with a defined alignment of neutral. It lacks any innate capacity to judge its own actions and perceive them as "wrong" — such a concept does not exist for the mountain lion, but the same cannot be said of a dragon beyond a certain age.
But right off the bat the rules seem to imply an intrinsic and pervasive evil in the species as a whole, quite possibly from birth, as racially encoded in the DNA as the dragon's wings are. And, to continue with the analogy, an older dragon can always choose not to fly, but it will still be born with wings.
A seagull, on the other hand, cannot ponder and reflect on its abilities and make a conscious decision that it shall never fly again. It just flies — because that's what it does; it is part of its very nature, like the murderous mountain lion.
So, a grey area in the canon I feel. My take on this is, admittedly, a bit of a judicious interpretation of RAW, but at the end of the day the DM is judge, jury… and of course executioner.
Just found this quote, which I presume (…) is from MM:
Always: The creature is born with the indicated alignment. The creature may have a hereditary predisposition to the alignment or come from a plane that predetermines it. It is possible for individuals to change alignment, but such individuals are either unique or rare exceptions.