Only everything important. Sorry to be flip but it is so hard to sum up. I knew absolutely nothing at age 24. I had no idea how to deal with people, how to manage relationships, my energy, or long term goals. How to balance and prioritize. I am nearing 40 and feel I am finally taking the first baby steps towards having even the slightest clue about what's going on. I'm pretty sure I will die before I get most of it straight. If you've mastered all the skills you need by 24 you've either set your goals on the wrong thing or you are a prophet. But that's just one man's humble opinion.