I think you’re conflating average and mean. When it comes to income average is typically median, which does include billionaires but wouldn’t skew the data due to their inclusion.

It’s a bit of both. Infants have more neural plasticity as others have mentioned, so when using the same learning strategy babies are going to learn better. That said, adults have a lot more knowledge and experience and are able to make connections to a lot more things; so there are methods aimed at teaching adults that are more effective than methods used to teach babies.

I can’t say for sure whether or not this particular study used proper testing, but as a whole introversion and extroversion is not pseudoscientific.

Jung wasn’t a good scientist, but he did a lot of studies and came up with a lot of theories, some of which happened to be at least partially correct. Also, you seem to be getting something mixed up because Jung defined introversion as an “attitude-type characterised by orientation in life through subjective psychic contents”, and extraversion as “an attitude-type characterised by concentration of interest on the external object”, whereas the more common energy focused definition is not from Jung at all - at least, as far as I am aware.

The big five personality traits, namely openness, conscientiousness, extraversion, agreeableness, and neuroticism have been shown to be consistent, even cross culturally.

There are limitations to that: like how it’s an empirical observation, that other personality traits exist that aren’t factored into those five, or that it’s possible there are a larger number of smaller subfactors that make up those five traits, but ultimately they are scientific.