The following is from the National Institute on Deafness and Other Communication Disorders:
How do speech and language normally develop?
The most intensive period of speech and language development for humans is during the first three years of life, a period when the brain is developing and maturing. These skills appear to develop best in a world that is rich with sounds, sights, and consistent exposure to the speech and language of others.
So! This helpful onesie will remind your lesser-inclined friends and family to treat their baby like a human being. When a fluently capable adult, the aforementioned baby will thank you.