Critical periods: Is it really impossible to modify a neocortex?

One of my ardors is adult learning, and I think if the learning process strongly correlates with the degree of neuronal binding, creating a new neuronal path and enhancing the strength of neuronal connections are pivotal to accelerate the efficiency of adult learning. However, I found a new research related to my interest. That is “critical periods” in our brain. Singer (2008) describes this idea in the following: “Once the respective developmental window close, neurons stop forming new connections and existing connections can no longer be removed” (p. 101). According to the author’s explanation, especially, our neocortex which is involved in higher functions such as reasoning, thoughts, and language can’t be modified once critical period is over. I’m looking for the way to modify the neocortex though it is said impossible to modify it after critical periods.
Reference

The Educated Brain: Essays in Neuroeducation