Once upon a time, nearly everyone worked until their 60s. Today some people retire at 35, others retire at 85. There's no right or wrong age to retire; the only requirement is that you must be financially independent.Financial independence is not some vague analog for "rich." It has a very specific definition: being able to cover your living expenses with the income from your investments.In other words, not having to work a job, in order to pay your bills. And you can reach it in a few short years if you're truly driven.It takes two core ingredients to reach financial independence young: will and knowledge. I can't help you with the will-you either have it or you don't. But the knowledge I can help with.Here are nine things you need to know, and more importantly to do, if you want to achieve financial independence by the time you reach 40.When you're young, an investment that drops in value is a temporary setback, not an emergency.That changes when you're no longer generating active income. When you first retire, you're vulnerable to sequence of returns risk: the risk of a crash early in your retirement depleting your nest egg beyond the point that it can recover.But for now, you're not dependent on your investments for income. So your goal should be high returns and maximum passive income (more on that shortly).As long as an investment isn't actively costing you money (such as a rental property with negative cash flow), and you have reason to believe it will rise in value in the future, don't stress about it. Just leave it be and keep investing while the market's down and assets are cheap!When I was in my 20s, I had the will but not the knowledge necessary to reach financial freedom. I made a lot of mistakes.That's the problem with being in your 20s-you think you know more than you do, because the universe hasn't had much of a chance to prove otherwise yet. It's a pleasant, if dangerous ignorance, and it doesn't last.Learn everything you possibly can from more experienced investors. Read every article you can; listen to every podcast you can. The sad fact is that new real estate investors make the same five or six mistakes, because they charge ahead without pausing to learn from others who made those mistakes.When I first started investing, I made the classic, all-too-common mistake of underestimating rental expenses. I didn't understand how rental cash flow works. It works through long-term averages rather than what happens in a typical month-a lesson I learned the hard way after tens of thousands of dollars in losses.