It is not an inborn thing with the child.

He gradually acquires it.

And it is not a sense of a distinct and separate self, first known and then compared with other persons.

On the contrary, it is gradually built up in the child's mind from the same material exactly as that of which he makes up his thought of other persons.

The deeds he can do he first sees others doing; only then can he imitate them and find out that he also is a being who can perform them.
So it goes all through our lives.