Calculus itself isn't inherently more difficult to learn that earlier math, but it is where many math students hit a wall because it has a lot of prerequisites; if you were a mediocre student in algebra and trigonometry, there is a good chance that you will have trouble with calculus because you never completely mastered other skills that you have to use over and over again when doing calculus.
There are bootleg videos on YouTube, and if you happen to be in New York, the public library system there records all major Broadway shows and lets people watch them for free.
Generalized Efficient Markets - if something is easy and obvious, someone else is already doing it.
You can get a lot of very tight feedback loops when playing video games, too...
Possibly Antonie van Leeuwenhoek's study of microorganisms; he made microscopes that were much better than anyone else's at the time, and he kept his methods secret and they weren't properly reverse engineered until the 1950s. (Conventional lens making techniques did catch up, and people like Robert Hooke had been investigating biology on micro scales, but he was probably a generation or so ahead of everyone else.)
Looks kind of cool, but won't you be reading and writing to disk an awful lot?
If there's a minimum voting age, I think there should also be a maximum voting age. Sufficiently old people suffer from cognitive and physical decline, frequently to the point of dementia, and can be as vulnerable to manipulation or coercion by their caretakers as children are to their parents. It seems entirely reasonable to me that people over 80 should not be allowed to vote.
I have a bit of a dilemma. What do you say to someone who says things like "I believe in ghosts because I see and have conversations with them"? (Not a hypothetical example!)