Speech sounds development in children has been studied and speech therapists use this information to determine if a child is producing sounds adequately for their age.
It is typical for children to say some sounds incorrectly as they learn to talk. They can pronounce some sounds earlier, like p, m, or w. Other sounds like z, v, or th take longer for them to acquire. Most children will be able to pronounce all sounds in English by the age of 8. A child who is not pronouncing these sounds by the by a typical age or is highly unintelligible may need to be evaluated by a speech language pathologist to determine if they have a speech sound disorder or they just need a little bit of extra support.