Let me qualify the above a little. You might ask, well what are the purpose of high-school teachers or university lecturers? Once we are given these tools shouldn't we just be given the books to progress on our own? Hopefully, teachers are people who can take a complex subject and guide us towards an understanding of that subject. To me teachers are like guides through a jungle of knowledge. We need a guide so that we don't get lost. For me, a teacher is part guide and part "composer of problems". I consider teaching to be guided problem-solving. When I am teaching a course, I pose problems, whether it be in Math or Chemistry, and guide the students as they try to solve those problems.
And as for your comment about "wasted time", I am not sure what to think about this. The difficulty with learning is that we need repetition and reinforcement. Yes, I see a lot of pointless repetition, but we do need some repetition. I am presently struggling with improving my chess skills, as well as trying to learn to program an Arduino. Mastering these skills require a lot of repetition in order to cement that knowledge. Wish me luck.
1. Young children seem to have the biggest capability in learning languages.
2. There's the John Stuart Mill example -- homeschooled by his father in the classics and philosophy. Interesting biography (Wikipedia has a fair account).
3. There are children who are naturally skilled in art, storytelling, maths, sciences, music, etc. An opportunity to work to those strengths might be useful.
I've also noticed that a short period of instruction at the appropriate time often seems disproportionately effective.
Is there any evidence that this is actually due to their capability? I would say it's far more likely it's their environment. IE, people are constantly talking to young kids. They simplify what they say to make themselves understood. And children have a huge incentive to learn to communicate - to get what they want from parents, or just understand the world around them. The same isn't true for a grown adult trying to learn a second language, in isolation, that they don't really need in their day to day lives.
Disclaimer: this is pure conjecture.
I'm also aware of a few other age/skill type relations.
Musical ability frequently manifests early -- by age 4-5 in numerous historical cases.
Talents requiring physical coordination -- ballet and gymnastics -- around age 9-10 by Russian tradition.
Maths and scientific curiosity perhaps early adolescence.
Complex logic and activities requiring a fully developed inhibitory function (avoiding reflex and rash actions) possibly early 20s.
I'm suspecting other forms of more synthetic (as in: assembling from parts) logic might not develop fully until the 30s or 40s. Other skills might similarly not develop until later.
Montessori schools are said to be better on this score, though I haven't looked into them.
Neal Stephenson's Cryptonomicon, The Baroque Cycle, and Anathem, for example, are all explorations of ideas. The first two of business, banking, and money, in contemporary and historical times, respectively. The latter covers a great deal of historical thought.
Better authors of topics often incorporate humour in their writing -- someting the past few generations of academic literature would do well to consider. OTOH, it's a cure for insomnia.