Quote from @Kaylee Loomer:
I’ve learned all there is to investing into Roth IRAs, Index Funds and ETFs
I would first caution against your notion that you've learned all there is to know about these topics. You may be significantly more knowledgeable about these topics than your average 18yr old, (or 40yr old for that matter), but there is always something more to learn and the only way to learn this additional stuff is to stay humble and engage in active learning techniques. If you think you already know everything then you will stop seeking out new information.
Retirement accounts are designed for....retirement, but that doesn't mean there aren't some good exemptions. You can withdraw penalty free up to 10k to purchase your first home, it also allows a 5k withdrawal for each parent after the birth of a child (per child). Most people will eventually both buy a home and have a child, so with this strategy you can let your money grow tax free in a Roth IRA and still have money to pay for these types of expenses.
You can also access your IRA funds before retirement age by taking substantially equal periodic payments (SEPP). Under this program you agree to withdraw a certain amount each year for at least 5 years or until you reach age 59.5. The rules to calculate the minimum required distributions is fairly complex, but there should be calculators out there to do it for you under rule 72T of the IRS. This can allow you to start withdrawing money slowly from your Roth accounts while you are still in your 30s or 40s.
Some people are interested in a 'lean' retirement, they aren't trying to get rich they are simply wanting to reach passive income of 40k, 50k, 60k whatever amount per year and then thats good enough and they quit their job as early in life as possible. If this is the case then a traditional IRA can be a fantastic option, and only you have quit your job start what is called a roth conversion ladder where you slowly convert your traditional holdings into a roth account.
While you can't withdraw the earnings from a Roth, you can always withdraw the contributions. So if you contribute 6k to a roth for 22 years until age 40, then you could likewise withdraw 6k per year for 22 years until age 62 to supplement any additional income (or simply withdraw all 132k all at once or any combination you want). 6k for 22 years at a 10% rate of return means a balance of 428k dollars at age 40. If you withdraw all 132k possible at age 40, you still have 296k in a retirement account earning tax free growth of roughly 30k per year.
Investing in index funds is great, but you can almost always get better returns by trading options. Options trading strategies such as the wheel strategy allow you to nearly universally beat the stock market average by a significant amount, (often 20-30% annual returns as opposed to 10% index returns).
As others have mentioned, the easiest and most surefire way to start getting ahead at age 18 is to buy a 3 or 4 bed property with something easy like a 3.5% down payment FHA loan, and then get roommates. The rent collected from these roommates will likely more than offset your entire mortgage payment allowing you to rapidly build up money to invest with. One of the biggest problems of wanting to invest at age 18 is that most 18yr olds don't have any money to invest with! So by focusing on income right now, either from your job, a side hustle, or getting roommates, it allows you to build up money so you can actually start investing.
I would highly suggest googling the FIRE movement, Financial Independence, Retire Early. There is an entire community devoted to this very topic and they can help you achieve your goals.