Question

the brain is shaped:
through external factors but only in early life
purely by genetics
by skill learning throughout most of a persons life

Explanation:

Neuroscience and psychology research shows the brain demonstrates neuroplasticity, meaning it can rewire and adapt through skill learning and experiences not just in early life, but across most of adulthood. It is not shaped only by early external factors or purely genetics.

Answer:

by skill learning throughout most of a person's life