In Spain used to be as low as 13 a few decades ago; but that law was obviously written before the rural exodus of inner Spain into the cities (from the 60's to almost the 80's), as children since early puberty got to work/help in the farm/fields or at home and by age 14 they had far more duties and accountabilities than today. And yes, that yielded more maturity.

Thus, the law had to be fixed for more urban/civilized times up to 16. Altough depending on the age/mentality closeness (such as 15-19 as it happened with a recent case), the young adult had its charges totally dropped.

Centuries ago, the limitation of most knowledge was the difficulty in discovery; once known, it was accessible to most scholars. Take Calculus, which is taught in every high school in America. The problem is, we're getting to a point where new fields are built on such extreme requirements, that even the known knowledge is extremely hard for talented university students to learn, let alone what is required to discover and advance that field. Until we are able to augment human intelligence, the days of the polymath advancing multiple fields are mostly over. I would also argue that the standards for peer reviewed whitepapers and obtaining PhDs has significantly dropped (due to the incentive structure to spam as many papers as possible), which is only hurting the advancement of knowledge.

More than that, as professionals' career paths in fields develop, the organisations they work for specialize, becoming less amenable to the generalist. ('Why should we hire this mathematician who is also an expert in legal research? Their attention is probably divided, and meanwhile we have a 100% mathematician in the candidate pool fresh from an expensive dedicated PhD program with a growing family to feed.')

I'm obviously using the archetype of Leibniz here as an example but pick your favorite polymath.

Are they fiendishly difficult or do we just need a von Neumann to come along and do what he did for quantum mechanics to them?