The first time I took an IQ test I was about 14 years old and I scored a 122, give or take a couple points. I scored a 640 on the math portion of my SAT when I was about 16. I'm stating these just for measurable purposes. My problem when I was younger was I could not answer the most complex questions, I always got confused while trying to answer the hardest questions and got stuck.
Now, at age 18, my IQ has gone from a 122 to a solid 145. You may think that number is outrageous, which it is, but I was tested at that. My math SAT score went from a 640 to a 800. When I approached old questions that used to stump me because I simply could not make any sense of it were seen in an entirely new light. Suddenly, the solution just became crystal clear. Complex problems were no longer an issue for me randomly. Everything just started becoming more clear, and everything just started making more sense.
The main thing that improved my IQ was constantly being exposed to complex information, and having someone with a high IQ explain it to me. The way they explained things changed my style of thought. Essentially, I was mimicking them in a "fake it til you make it" style. I now would break complex information into smaller pieces, and then connect them altogether at the end to form a solution. Not only that, but I focused on developing one of my personality functions (Ni) by simply understanding it more but that's an entirely different subject. I also stopped thinking out loud and pausing my thought process to make sure everything makes sense, instead I just followed my gut on all decisions. I can no longer explain most of my reasoning to someone else, it usually takes me a while to backtrack my thought process. I USED to explain my thought process to myself, which ended up in just me getting confused in the middle of problem solving so I stopped doing that.
Now, the argument here could be that my IQ did not increase, but the innate IQ I possess revealed itself because I altered my mindset. However, I don't understand how the change could be so drastic in just 2 years. Something had to improve it.