Defining perfect pitch is not a straightforward matter. The correct term is absolute pitch, or the ability to identify or generate any pitch without a context. This is to distinguish it from relative pitch, which is the ability to recognize or generate a pitch after being given a reference pitch. For example, if you played a C on the piano and named the note, then you played a G and asked what it was, a person with relative pitch would be able to recognize the G by comparing it with the C. This skill can be developed at any age.
What we generally refer to as perfect or absolute pitch is the ability to sing a G out of the blue, any time, on demand, without any reference pitch. On one hand, it is not entirely accurate to talk about developing this skill, because all babies are born with it. On the other hand, if it is not maintained, by about the age of one it disappears. On the third hand, it is not enough to be able to hear the differences between the pitches, which all babies do: it is necessary also to be able to attach the correct name to each pitch, which must be learned. Babies acquire this skill the way they acquire their native language: by extensive exposure to the association between the object (sock) and the corresponding word (“sock”), and in the case of pitches, between the sound (sol) and the corresponding word (“sol” or “G”).
The reason most of us don’t have perfect pitch is that our parents didn’t point out to us the pitches of the sounds in the environment. It is possible to argue that there is a reason why our mothers were able to point out to us the color of the tomato and of the grass but not the pitch of the birdsong or the thunder. Being able to differentiate between colors is a selected trait; those who couldn’t tell the color of the mushroom ended up eating the poisonous one. Knowing the pitch of the cat’s meow has no consequences for survival. This may be perfectly true, and it may explain why so few of us have perfect pitch. We’re in a vicious cycle. The fewer parents have perfect pitch, the fewer are able to help their children retain it. But it does not disprove the mechanism by which it is achieved and achievable, or the benefits that can accrue to those who have it, that the brains of those who have perfect pitch differ from the brains of those who do not. Going one step further, it is reasonable to argue that retaining perfect pitch is an important element in achieving native fluency in music. Indeed, it is almost an automatic byproduct of the exposure that eventually leads to native fluency in music.