What does Judy Heumann mean when she says, "the barriers we're facing, in many ways, are external to us"?

Why it's time to lay the stereotype of the 'teen brain’ to rest 2. part b: which detail from the text best supports the answer to part a? a “brain deficits don’t make teens do risky things; lack of experience and a drive to explore the world are the real factors.” (paragraph 1) b “according to this theory, the prefrontal cortex, the center of the brain’s cognitive-control system, matures more slowly than the limbic system, which governs desires and appetites including drives for food and sex.” (paragraph 5) c “in experiments that mimic the well-known marshmallow test, in which waiting for a bigger reward is a sign of self-control, adolescents are less impulsive than children and only slightly more so than adults.” (paragraph 12) d “the majority of adolescents do not die in car crashes, become victims of homicide or suicide, experience major depression, become addicted to drugs or contract sexually transmitted infections.” (paragraph 14)