At what age a child speaks a language perfectly?

Difficult issue: Acquisition of understood speech and what you consider as "perfect" are separate issues. Most toddlers produce speech that is 50% understood by strangers at 2 , 75% at 3 and 95% at 4. Some blends and other issues can persist until school age.

3-7: Children usually begin by speaking like their parents or caregivers, but once they start to mix with other children (especially from the age of about 3 years) they start to speak like friends their own age. They will develop their own accents and they will learn the languages they think they need. By age 4 they should begin developing sentences. Each child is different and develops differently.