They're not talking about a critical period for learning second language, but for a first language.

We have a few case studies of extreme neglect where children were kept alive but never spoken to or around by their caregivers. These children do not learn any language, and after a certain point (the critical period) they don't seem to be able to ever truly learn a language anymore. "Genie" being one of the most well-studied cases. She was kept restricted on a potty chair for 13 years, in a dark bedroom away from the rest of her family. She was fed, but that was about it. She was eventually rescued, but was never able to master English grammar. She knew some words, and could somewhat use them to communicate, but was never able to convey more complex ideas.

What you are talking about is a so called "sensitive period." It is easier to learn a second language earlier in life, but not impossible to do so later.