I feel like it's the wrong question. I don't think anyone would really advocate postponing responsibilities until the brain had reached maximum maturity. The responsibility question's function signature is wrong: it's returning a boolean when it should be returning a float.
This discussion highlights why these questions are so difficult and frightening: ethics prevents good science.
Randomly sample a bunch of kids, hook them all up to monitoring devices, put them in different schools and societies that involve transitions into adulthood at different rates and onsets, set up a control group (Lord of the Flies setup), do the longitudinal study, and write it up. An associate professor and his 5 grad students would get a bunch of solid papers out of it. Theorists would have fun defining "optimality" in this context (to be fair, the philosophers have been gnawing at it for a while..)
This is unethical .. right? Then, you have to ask yourself: wouldn't it be more ethical to do the study, figure out how best to raise kids, and let all human young for posterity (or at least 'til the results are invalidated) benefit from the wisdom?
It would only be more ethical if your ethical system was based on a utilitarian viewpoint.
In contrast, most capitalist democracies (all that I can think of anyway) are centered on individualism... the only ethically allowable sacrifice is the voluntary sacrifice... and children (anyone under 18) are not considered sentient enough to make decisions about their own lives and futures.
You prove him right: by assuming underage people aren't sentient enough, ethics prevent us to check. Not necessarily bad, but definitely restrictive, and not good science™.
But is it because young adults these days don't have to grow up as much as teenagers had to say a century ago? You can effictively stay a child emotionally for a very long time.
As far as I can tell, that pattern holds for at least another decade.