If immigrants "reset" to a toddler's level, at what age does a native speaker actually stop being a "linguistic toddler" and reach adult proficiency?

There’s a common saying about immigrants: in the first few years, your language level is like a toddler's, then it progresses to a school-level, and so on.

This made me think about native speakers in their own country. At what age is a person considered to have acquired enough vocabulary and grammatical structures to actually navigate life? It obviously doesn't just happen overnight on their 18th birthday.

Is there a specific developmental milestone or age (12? 14? 16?) where a person’s linguistic "toolbox" is essentially complete for daily adult life, even if they continue to learn niche vocabulary later?