Your brain is the most susceptible to change during major periods of development like adolescence and young adulthood.

在青春期和青年期等主要发育时期，大脑最容易发生变化。

A " stage of development" is a period of development, when something is growing.

“发展阶段”是发展的时期，当事物正在成长时。

To conclude, English mainly underwent three periods of development.

总之，英语主要经历了三个发展时期。