I think it depends on the individual, but I have a notion of learning ability as being rather like filling assorted vessels with water on a beach.
Think of each vessel as labelled with one endeavour - Mathematics, Languages, Music, Sport, whatever; and you can sub-divide these too into internal categories. You can fill each container but only to its maximum. Thereafter you can try to add more but it just overflows and the addition is lost in the sand.
Now, I am not a neurologist so I don't know if my model is correct. The human brain is vastly more powerful than strictly necessary for running our bodies to keep us alive: at organ level there's not a lot of difference between a human and another primate, or any other mammal, in how our bodily functions function; but we have far more brain processing and memory.
A lot of it is connected with speech and emotions, but much of that extra capacity is available for all manner of things of no strictly biological importance. It would be a poor life indeed without culture, say, but we won't die through not learning to be fluent in Mandarin Chinese, singing a lead role in a Wagnerian opera or being an international sports star.
Yet I think we still have individual limits on what we can achieve even in areas where we would like to do well. Some will learn Chinese - others might just about manage "Thank you" in French. Some will move the audience to tears with the immolation aria from Gotterdammerung - others would hardly give Beyoncé or Madonna a run for her money. For all the Wayne Rooneys there are many would-be footballers who have to remain content with their Sunday-morning amateur games - though at least they are keeping fit! The would-be ones may dearly wish to reach such heights - but just don't have the basic ability.
As for "doors"....
I don't know but I believe yes, doors do exist and can be locked, is a sort of self-defence. The mind seems to cry "Enough! I cannot take any more attempts to be multi-lingual / an opera soprano / Premier League soccer player!"
Some depends on how the topic or subject is taught. If your teacher is unable to explain the matter you won't learn it. However, even if you have a very good tutor there is still no guarantee you will learn it.
I have quite small mental buckets, and some have stoppers (as in your "doors") that are in, waxed, wired and immoveable. I left school with only modest exam results simply because I was slow to learn, and find many things very hard to learn too. Some of my teachers were poor at teaching, but mainly, I was incapable of learning despite allegedly having a high IQ!
In recent years I have tried to learn various forms of software known generically as CAD (Computer Aided Draughting), used for various types of technical drawing. This is for hobby purposes, not a career or anything. I have about given up, recognising that it too, is a subject for which I have no mental capacity - the door is closing, and once closed my brain will lock it and throw away the key.
So to answer your question in a few words: a) Not very teachable. b) Yes, many doors now bricked up. c) Flexible? I try to be, but flexibility is not the same as learning ability. d) Why? Individual reasoning, understanding and memory power limits.
Once again you have awed me with the thoughtfulness and depth of your reply to my question Durdle. For which I thank you. When I was young I skipped a few grades in grammar school. My mind was a sponge. I loved learning more than anything. I got bored with summer vacation after the first week and counted the days till I could go to school again. I'm that same person. I love to learn new things. BUT my capacity for doing so is not the same as it was when I was young. It simply isn't. There are some fields I'm drawn to and some in which I have zero interest. When I was young I was interested in everything. Maybe because we have less energy to devote we become more selective as we age. Or lazier. You know the saying..."If at first you don't succeed try try again"? I don't. I just move on. Don't want to spend the time it would take because I am more aware of the fleeting nature of time. Most of my life is behind me. I want to make the most of what is ahead and get the best return on my investment. Nice seeing you again. Happy Friday! :)
You know what's weirdly funny Zack? At least to me? Math was my least favorite subject. I had the hardest time with it in school. Guess what I ended up being? An Internal Auditor/In-House Accountant! I LOVED it! Who knew? It just seems so strange to me to have been drawn to that. I have an affinity/love for numbers. Also researching things. I don't mean higher math like physicists would need but the basics that you need to analyze things and figure out what's wrong and how to fix them. I LOVED audits! Go figger! I think aging has a lot to do with it. At least for me. All the electronic thingies leave me cold. I have no desire to learn how to operate/use them. The younger me would have lapped it all up. Thank you for your reply and Happy Friday. Oh. And when Jim wants to explain stuff to me..you know, guy stuff about how things work or should? Honestly I tune out. I know. That's not very polite! :(
It does become harder with age to learn anything to a high level, but not impossible.
I think we need a combination of innate ability and a good teacher who can see why we find it hard. I'd be stuck with all these electronic gadgets about now, too. For part of my working life I had occasionally to stay in hotels for short spells, and was rarely able to watch the television in the evenings because I have no TV at home so had no idea how to use the remote controller. No instructions in the room because the hotel owners all assumed "everyone" knows how to use them.
I've tried to learn some specific technical software I thought I could use to support my hobbies, but it proved beyond me. Thinking about it, I realised the various versions I'd tried must demand a certain amount of basic knowledge in the software's overall principles before you learn the individual programmes' own characteristics. I've no idea how you learn these principles except in expensive formal courses, and the 'Help' options available are all just aide-memoires for the particular software, not very helpful to beginners.
I think one problem at school was that each subject was taught for an exam at the end of the year or set of years.
French was not a language millions live their daily lives by, apart from some vague stereotyping in the text-book that led one to think the French all live in Paris, eat "biftek at frites", drink wine and read high-art literature. (The books presented excerpts from these novels, as practice.) It was just an interminable struggle to remember the gender of nouns and learn obscure verb endings. The idea I might ever visit that country, its nearest coast only about 80 miles across the sea from my own home sea-front, never entered my head. I did, years later, several times too.
Similarly, Maths was not a skill used everyday in all manner of trades and professions but a set of abstractions I found very difficult indeed to grasp. I have though spotted in AM and in Wikipedia's "Answers-dot-com" site, what appears to be a significant difference in approach between the USA and UK in teaching Mathematics in school. In the UK, Maths is a cohesive, single curriculum subject, albeit broken by syllabus into its various topics like algebra, calculus and trigonometry. It looks as if US schools don't teach "Math" as a single curriculum subject, but treat each mathematical topic as a subject in its own right. It must work, but I find it hard to see how, because as you advance your mathematical knowledge and start to meet examples from science and engineering, you find more and more examples of individual problems combining techniques from different topics - and almost all mathematics relies on Algebra.
Many British parents of my generation have been baffled when their children ask for help with arithmetic homework, as their children are being taught very strange, fancy and more complicated, new methods the parents have never encountered. And that is in problems as simple as long-multiplication! Some schools have run special evening-classes to help such parents. Whoever thinks a method that has been used for centuries because it is simple and efficient, is suddenly out-of-date and no longer "correct", and why, I have no idea - perhaps they haven't either.
Durdle 