One of the biggest retirement mistakes you can make is not realizing what you don’t know.

I regularly hear from people in or near retirement who misunderstand how Social Security works, dramatically underestimate life expectancies or fail to plan for big expenses, such as long-term care or taxes.

These aren’t folks looking for advice. They’ve already made up their minds and want to argue about financial planning precepts, such as when to take Social Security or how much retirement is likely to cost. But what they think they know just isn’t so.

The reality is that most people don’t get good, objective financial advice before they retire, says actuary Steve Vernon, consulting research scholar at the Stanford Center on Longevity.

Many people simply wing it, figuring that if they have a Social Security check and a little savings, somehow everything will work out.

Unfortunately, retirement is complicated, and your decisions can have irreversible consequences. Talking with a professional — ideally a fee-only financial planner — could save you from a costly mistake, including any of the following.

THINKING YOU’LL DIE YOUNG (OR AT LEAST EARLY)

If you die early in retirement, your worries about paying for it are over. Live longer, though, and you easily could outlive your money. That stacks the deck in favor of waiting to start Social Security, since each year you put it off from age 62 to 70 increases your benefit by about 7% to 8%. That’s a guaranteed return on a stream of income that you can’t outlive or lose in a stock market downturn.

Plus, you may live longer than you think. The average U.S. life expectancy is just under 79, but that’s from birth. If you make it to 65, you can expect to live another 20 years or so. Half of all women currently in their mid-50s will live to 90, as will 1 in 3 men, according to the Society of Actuaries.

People with healthy lifestyles and more education tend to live longer than average.

IGNORING YOUR SPOUSE

Speaking of Social Security: When one spouse dies, one of the couple’s two Social Security checks goes away. The survivor has to get by on the larger of the two checks.

It’s important to maximize this survivor benefit by having the higher earner delay filing for Social Security as long as possible. Also, married people who will get a pension should strongly consider a “joint and survivor” option that allows payments to continue for both lives.

CARRYING DEBT INTO RETIREMENT

If you’re wealthy, having debt may not be a big deal — you have plenty of income to make the payments, and your investments may be earning more than you’re paying in interest. If you’re not rich, though, you may be pulling too much from your savings to service the debt. That could increase the chances you’ll run out of money.

Big withdrawals from retirement funds also could push you into a higher tax bracket and increase your Medicare premium. Give yourself some options by planning to have debt paid off by retirement, but consult a financial planner before you tap retirement accounts to pay off any big debts, such as a mortgage.

FAILING TO PLAN FOR LONG-TERM CARE

If there’s anything people want to ponder less than death, it’s decrepitude. Yet someone turning 65 today has a 70% chance in the future of needing help with daily living tasks, such as bathing, eating or dressing.

Family and friends will help some, but about half will incur costs for long-term care — and 15% will incur costs of $250,000 or more.

Long-term care insurance may be one solution, or you may want to earmark certain investments or your home equity.

ASSUMING YOU CAN WORK LONGER

About half of retirees report leaving the workforce earlier than they had planned. A few get lucky, thanks to windfalls or strong stock markets. Many more retire because they lose their jobs and can’t find replacements or because of ill health (their own or a loved one’s). Working longer can help you make up for not saving enough, but it’s not an option you can count on.

PUTTING OFF RETIREMENT TOO LONG

With all this gloom and doom, now I’m telling you to hurry up already? This bit of advice is for my fellow ants living in a grasshopper world: Sometimes, the grasshoppers get it right. Time, good health and energy are all finite resources.

Spend a few hundred bucks of your hard-earned savings on a fee-only financial planner — and find out if it’s time to start living the future you’ve been saving for.