While it is normal for young children to simplify their speech when they are learning to talk, there are typical ages where certain sounds are expected to be mastered, and certain errors are expected to have disappeared. Sounds produced in error beyond what is considered age appropriate are referred to as articulation or phonological errors.
Articulation errors are motor-based, meaning the error occurs due to a difficulty moving the articulators such as the tongue or lips. A child needs to be taught where to physically put their articulators to produce the sound correctly.
Phonological errors occur around a pattern of speech sounds, for example sounds made at the back of the mouth (e.g. /k/ and /g/) are changed to sounds made at the front of the mouth (e.g. /t/ and /d/) when they occur at the start of words (e.g. ‘key’ to ‘tea’, ‘got’ to ‘dot’). It is common for a child with this phonological error (referred to as fronting) to produce /k/ and /g/ sounds in other word positions (e.g. ‘chicken’, ‘bag’) but have mis-learnt the pattern for these speech sounds at the beginning of words and need to be taught a new pattern to produce the correct sounds in the correct spot.