You’re thinking about the “Critical Period” theory. That’s the theory that says that a child has a window of opportunity, or a limited time, to learn language or concepts before the window “closes.” Some people believe the ability to catch up is forever lost, while others believe that the ability to catch up is just more difficult, but not impossible.
The MAIN thing wrong with that theory is that it is ultimately rooted in the results of two children that were separate from society for years, and upon being found, were taught language and proper behavior. Both kids “failed” to re-integrate into society and acquire language. One is “Genie”—she was chained to a toilet for years and the “wIld boy of Aveyron” who lived in the woods with animals. Genie never acquired full use of language- but she had been severely abused before she was found and severely malnourished, so there is no way now to identify what kind of other cognitive problems she had, but she’s often cited as an example of a “closed critical period window.” The wild boy was found and given to a very abusive teacher whose notes say he never learned to behave or even make eye contact. Careful review of those notes today show that the boy most likely had some kind of autism/Aspergers, but again, he is often cited as an example.
I have an example of how the closed window is false— I was a teacher in Arizona for a while and my middle school got a deaf girl who had been homeless and abandoned in Mexico. She lived on the streets, foraging and surviving. She had zero access to spoken languages- so knew no Spanish or English, and had no access to any signed languages, so knew no Mexican or American sign language. The social workers found her and brought her to our school at age 12. Theoretically, she should have had ZERO ability to acquire language, but within 3 years she was able to catch up considerably, could communicate and read/write at the level of a 8 year old (meaning, in 3 years she covered 8 years of language acquisition). She was still behind, but helluva lot better than where she was when she started.
Today’s research shows that children under 12 have much more brain plasticity and flexibility, but that the brain doesn’t really stop developing until age 25. After that, the brain STILL can develop new neural pathways in many cases. The brain is so complex that every single individual has different results.