At age 19, George Boole did a startup: he started his own elementary school. It seems to have been decently successful, and in fact Boole continued making his living running (or “conducting” as it was then called) schools until he was in his thirties. He was involved with a few people educated in places like Cambridge, notably through the local Mechanics’ Institute (a little like a modern community college). But mostly he seems just to have learned by reading books on his own.
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It wasn’t long before he was interacting with leading British mathematicians of the day, and getting positive feedback. He considered going to Cambridge to become a “university person”, but was put off when told that he would have to start with the standard undergraduate course, and stop doing his own research.