If you're not on Kylie Jenner's financial wavelength, making smart money decisions in your 20s, 30s and 40s is imperative if you want to retire as a millionaire. In fact, it's imperative if you want to retire at all. In 2018, the rate of adults ages 65 and older who filed for bankruptcy more than doubled since 1991, according to AARP's "Consumer Bankruptcy Project" report. The data further reinforces a harsh reality: Many Americans are not prepared to finance 30 years of retirement. Retiring as a millionaire is easier said than done, but it isn't impossible. There has been a significant boost in the share of millionaire retirees in the U.S. since 1989, now accounting for one out of every six retirees, according to United Income's "State of Retirees" report. Here's what to prioritize in your 20s, 30s and 40s if you want to increase your chances of retiring with $1 million:

In your 20s...

1. Take advantage of compound interest With compound interest, any interest accrued earns interest on itself. In his book "Financial Freedom," self-made millionaire Grant Sabatier explains that the younger you are, the more time you have for your money to grow. "If you keep saving and investing, your net worth will keep growing, and because of compounding, the growth will accelerate," he says. Contributing to an employer-sponsored 401(k) plan is the simplest way to get started. The general rule of thumb is to save 20% of your income. But financial goals, budgets and means vary from person to person, so if you have relatively low living costs or a high salary, you can retire earlier by contributing up to the IRS limit of $19,000 (for 2019). Don't be discouraged if you're on a tight budget — saving something is better than saving nothing at all. At the very minimum, take advantage of your entire employer match (some companies offer a match of up to 6%). If a 401(k) plan isn't an option for you, look into other tax-advantaged retirement vehicles, such as a ROTH IRA, a traditional IRA or a SEP IRA. 2. Avoid (or pay off) credit card debt In 2014, Mark Cuban told Business Insider that the one thing he wishes he had known about money in his 20s is that credit cards are the worst investment that you can make. "The money that I save on interest by not having debt is better than any return I could possibly get by investing that money in the stock market," he said. "I should have paid off my cards every 30 days." To avoid racking up credit card debt, keep living expenses to a minimum. Consider living in a studio versus a one-bedroom. Take public transportation. Bring leftovers for lunch. Negotiate your bills. Don't blow off your bonuses or raises on extravagant purchases; use that money to pay off your highest rate of debt instead. It's also a good idea to open a high-yield savings account to start building your emergency savings. This will provide you with a favorable alternative to credit cards when money gets tight. 3. Maintain a healthy credit score Building a strong credit score in your 20s can pay dividends for the rest of your life. Healthy scores open the door to benefits like higher credit limits, lower interest rates (on mortgages and auto loans) and lower premiums (on auto and home insurance). Oh, and the bragging rights are also nice.

Building a strong credit score in your 20s can pay dividends for the rest of your life.

Make it a point to consistently pay your bills on time and keep your balances low. Generally, you should aim to keep your credit utilization ratio below 30% (a.k.a. only use less than 30% of the total credit available to you).

In your 30s...

1. Boost or develop additional income streams Diversification is a fundamental rule when it comes to intelligent investing, especially in your 30s — after you've graduated from college and settled into a stable career. Hopefully, you'll also have time to explore other hobbies and skills. Instead of spending an entire weekend watching Netflix, use that free time to develop additional income streams. Pick up a side hustle: Rent your car, sell things you don't need, take on some freelance gigs. The opportunities are endless. If none of the above sounds appealing, you should still be active in trying to boost your earnings. That might mean asking for a raise, looking for another job where you can negotiate a higher salary or opening an investing account (Warren Buffett recommends investing in a low-cost index fund). It's a bad sign if you're in your 30s and still making nearly the same amount you did in your 20s. 2. Increase your retirement contributions It's fine to have a few mishaps in your 20s. But by the time you hit your 30s, you're nearing your peak earning years — meaning that you'll need a solid grasp on where you stand financially. That's why it's important to do a thorough review of your retirement accounts to make sure you're on track to hit that million-dollar finish line. If you're already enrolled in a 401(k) plan, this is an excellent time to increase your contributions. It's also smart to consider opening more than one retirement savings account, such as a Roth IRA, traditional IRA and/or health savings account. 3. Have the "money conversation" with your partner This one isn't exclusive to 30-somethings. If you're in a serious relationship — no matter what your age is — set aside time to talk about money with your partner. Ask questions about each other's financial habits and goals for the future. If you have different views on money, working out those disagreements is essential to a long and happy relationship. And, if you're serious about getting married, the prenuptial agreement is one of the most important financial discussions you'll ever have with your partner. Not fun, I know. But people in relationships that start out seemingly idyllic sometimes part ways. Prenups aren't for everybody, so carefully review your financial situation and discuss whether it makes sense to sign one.

If you have different views on money, working out those disagreements is essential to a long and happy relationship.

Keep in mind that the average cost for a divorce attorney in the U.S. is $250 per hour, according to a 2018 survey by the legal site Nolo. The average couple spends up 50 hours ($12,500) over a period of 10.7 months. In states like California and New York, legal fees can be as high as $25,000 for divorces involving children. Depending on your situation, having a prenup can shorten the time it takes to resolve the case.

In your 40s...