Unfortunately, you have not attached the text that should be referred to support the answer as evidence. However, I can answer the first part of your question. You say "malleable" yet this does not refer to the physical moulding of the brain and rather, refers to how teenagers are highly influenced by their surroundings because of their brain development. Before adolescence, a teenager's brain is not fully developed and is undergoing rapid changes, referring to its shape, functionality and size. Teenagers are extremely vulnerable and impressionable because the structures in their brains used to manage decision making and emotions are still developing. As a result, teenagers are susceptible to anxiety and stresses, more moody from their hormones and "malleable" in the terms that they will be more likely to fall victim to peer pressures. As teenager's brains are still developing, they are unable to make sensible and mature decisions all the time.
I hope this makes sense and helps! : )
