When do speech and language begin?
Infants start learning in the womb, where they hear and respond to familiar voices. The fastest learning occurs from ages 2 to 5 years.
Speech and language milestones help tell whether a child is developing as expected. Milestones are certain skills, such as babbling, saying "mama" or "dada," or putting two words together. Usually, a child needs to master one milestone before reaching the next.
Babies usually start cooing at around 2 months and are babbling by about 6 months. A child usually speaks in gibberish, called jargon, by the first birthday. At 15 to 18 months, a typical toddler understands much more than he or she is able to put into words. Starting around 18 months, many children have a burst in talking. By 24 months, children tend to use at least 50 words and are also starting to use two-word phrases.
Keep in mind that the age at which children reach milestones varies from child to child. Some children, especially girls, are advanced. Others develop more slowly.
Source: http://www.webmd.com