Final answer:
The phonological simplification process, which typically resolves by age 4 for /l/ and 5;6 for /r/, is a normal part of language development that includes omission and substitution of sounds like in 'wabbit' for 'rabbit', known as gliding.
Step-by-step explanation:
The phonological process referred to in the question, which occurs up to about age 4 for /l/ and up to age 5;6 for /r/, is known as phonological simplification. This is a normal part of language development where children simplify adult speech as they learn to talk. They may omit sounds (omission), replace them with easier ones (substitution), or simplify complex sounds into simpler ones. The specific process affecting /l/ and /r/ is a type of substitution where children might use the sound /w/ for both /l/ and /r/ (gliding). For example, they may say 'wabbit' for 'rabbit' or 'weaf' for 'leaf'. This developmental process helps children gradually acquire the phonemic inventory of their language, but it becomes a concern if it persists beyond the typical age of phonological development.