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How much do you know about your credit scores? Those three-digit numbers are tied to our financial lives, yet many young adults haven't given them the attention they deserve.

Your scores can play a role in your ability to rent an apartment, qualify for a loan or even get a job. They can also affect how much you'll pay on interest charges, insurance and even cellphone contracts.

Make building stellar scores a priority while you're young and you could actually save hundreds or thousands of dollars over your lifetime. However, if you don't take your credit seriously, bad scores -- or even nonexistent scores -- will cost you.

Who's keeping score?

Your credit scores are basically used to predict the possibility that you won't pay your bills. Scores are compiled by Fair Isaac and are sometimes called FICO scores. The top possible number is 850, but topping 800 is probably unrealistic. A median score usually falls in the 720-to-725 range, meaning half of consumers fall above that point, half below. Even if you haven't given your FICO scores much thought, there are plenty of others who have or will, so you'll want to aim for the mid-700s to make the best impression on:

1. Lenders. This group is the one most people associate with their credit scores. Having a good rating can help you qualify for the best rates on a mortgage, car loan, credit card and even a small-business loan if you've got that entrepreneurial spirit. Nonexistent scores can make it impossible for you to qualify for a loan or credit card.

2. Insurers. The majority of auto insurance companies use your credit scores when determining your rates, and the practice is also common among home insurers. A survey by Consumer Reports among eight popular auto insurers found that drivers with top scores pay up to 31% less on their premiums than if credit scoring wasn't factored in, while those with bad scores pay as much as 143% more.

3. Landlords. Increasingly, you may need good credit scores to rent an apartment. Landlords view your credit rating as a measure of your responsibility to pay bills on time. If your rating is below par or you don't have credit scores yet, you may have to find a friend or relative to co-sign your lease, or you could be required to pay a higher rent or security deposit.

4. Employers. When you're applying for a job, potential employers can pull your credit reports as long as they notify you first. And, in fact, about a third of them do, according to the Society for Human Resource Management. Why? Bad credit can be a signal of irresponsibility, or employers might be worried you'll spend more time fretting about your financial woes than concentrating on the job.

5. Cellphone carriers. Even cell phone service providers may check your credit before signing you up for a plan. They want to make sure you're responsible and will pay your bill each month. Some utility providers may pull your reports as well. If you have credit issues, you may not qualify for the best plan rates, you could be required to pay a deposit, or you could get turned down.

True cost of your scores

So, how much do your credit scores affect your finances? Say you have two friends, Jim and Mark. Both took steps right out of college to start building a credit report by getting their first credit cards and an auto loan. Jim made all his payments on time, never maxed out his credit cards and often paid more than the minimum required. Mark, however, frequently paid late, overextended his cards and applied for new credit to bail him out of his mismanaged debts.