An articulation disorder is an inability to correctly produce speech sounds (phonemes) due to imprecise placement, timing, pressure, speed or flow of movement of the lips, tongue or throat. (The Free medical dictionary)
Learning to articulate correctly is a process that requires time in children. As a rule of thumb, one expects that a child should be understood by people outside the family by the age of 4. Not all speech sounds will be correct at this age, but some will continue to develop until the age of 7-8 (e.g.[r] and [s]).