The Financial Times had an article with the above question. The author asked various people to answer the question.
While I was not interviewed, if I was asked this question I would say that I wish I had known in 1975, when I was 18, that I was raised in a cult and that the continued focus on Armageddon over my lifetime was a cruel joke. The rest I could have figured out if I had only known that I was taught a lie my whole life.
What do you wish you had known at 18?