By what age should a child's speech be 100% understandable?

A child's speech should be 100% understandable by the age of 4 years. At this stage of development, children typically have a vocabulary of about 1,000 words and are capable of forming sentences that reflect more complex ideas. While they may still occasionally mispronounce certain sounds, generally, their speech becomes clearer and more intelligible to listeners who are unfamiliar with them.

By age 4, children are also more adept at using language for communication, which involves not just speaking but also listening and understanding others. This is the age when their ability to connect words and phrases to convey meaning significantly improves, enhancing their overall conversational skills. As they continue to grow and develop beyond this age, their speech and language skills will become even more refined.