If you could send a short letter back in time to your younger self (at any age), what would it say and at what age would you want it delivered? Seven successful business leaders were asked this question, and their candid answers reveal smart career advice we can all use. For example, "Get serious about learning ... and don't be so arrogant," is the message that Dan Lok, a serial entrepreneur, best-selling author, and two-time TEDx opening speaker, would tell his younger self. Visit Business Insider's homepage for more stories.