Question

As the infant grows, his mental ability

Answer 1

will also grow but sometimes it might not in some special cases

Answer 2

As the infant grows, his mental ability increases.

He slowly learns language, signs, gestures, ways to speak and communicate from his surroundings.

His mental ability allows him to perceive things from his surrounding environment and helps him grow as an individual.

But in some special cases his mental ability may not increase like others specially when he is suffering from any mental disorder.