Infant

Infant: A child up to 2 years (24 months) of age.

The word "infant" came from the Latin infans which was derived from in-, not + Fari, to speak = not to speak, speechless. The idea was that, since the ability to speak was thought to arrive at the age of two, younger children were infants.

By an odd twist, some adults may only show their infantile attitudes when they open their mouths to speak.