Earlier quoted context omitted.
Fluid intelligence drops with age: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4906299/ You weren't an idiot as a teenager. You were just uneducated and inexperienced. Personally, my decaying mental performance over the years is almost palpable. I feel like I am half-dead now compared to who I was 20 years ago. Sure I was nuts due to the powerful emotions, but that just means I was alive, not a talking corpse that I…
Yes but when does it start dropping? Not likely in the teens. As I'd always heard and other commenters also suggest, peak brain power is usually ten years later than this article proposes. Brains are still developing in our teens, and are not at their peak yet.
Brain morphology is irrelevant. "Development" might just mean aging. Measurable performance is all that matters.