It is typical for children's speech sounds to develop as they grow.  Typically, a baby will begin to "coo" within the first few months of life.  In a short time, they are babbling.  First, babies babble in repeated syllables (i.e. "mamama"), and later in syllables that change (i.e. "mamemo").  First words tend to appear around a child's first birthday, with a rapid growth beginning around 18 months.  Children will often use learned "true" words mixed with "jargon"/babbling which begins to mimic the patterns of adult speech.  In time, children, will begin to use most sounds correctly, and will be easily understood by others.  Children often follow typical "patterns" of speech sound development called "phonological processes" which are a normal part of the speech development process.  Some patterns can include sound or syllable substitutions, omissions, and additions.  These eventually begin to disappear, as more intelligible speech emerges.  Most of these patterns begin to subside by ages 2 1/2 to 3 1/2 years of age, but a few can last until age 5 or longer.

If you are concerned with your child's speech sound development, please discuss with your physician, and seek an evaluation from a licensed speech/language pathologist to determine whether a problem exists.