From The American Speech Language and Hearing Association (ASHA):
Speech sound disorders is an umbrella term referring to any combination of difficulties with perception, motor production, and/or the phonological representation of speech sounds and speech segments (including phonotactic rules that govern syllable shape, structure, and stress, as well as prosody) that impact speech intelligibility.
Known causes of speech sound disorders include motor-based disorders (e.g. childhood apraxia of speech), structurally based disorders and conditions (e.g., cleft palate), syndrome/condition-related disorders, and sensory-based conditions (e.g., hearing impairment).
Speech sound disorders can impact the form of speech sounds (articulation disorders) or the patterning of speech sounds within a language (phonological disorders).
As children develop, they often experience various sound substitutions and distortions of speech sounds that are typical for their age, due largely in part to development of their oral structures, motor planning, and their brain development. Many children also experience phonological processes as they develop, including final consonant deletion (“cuh” for cup), fronting (“tat” for cat, “doat” for goat), and weak syllable deletion (“nana” for banana). Children develop each speech sound at a different age, most being mastered by age 5 or so. If articulation errors continue past a certain age, or if a phonological process has not been eliminated by the expected age, speech intervention may be recommended.