By the time many people reach age 70, they've already been collecting Social Security for years. But if you're nearing your 70th birthday or recently turned 70 and haven't started benefits yet, there's a key Social Security rule you need to know about.

There's no sense in delaying Social Security past age 70

The earliest age you can claim Social Security is 62. And you'll get your monthly benefits without a reduction at full retirement age, which is 67 for anyone born in 1960 or later.

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But each year you wait to file past full retirement age boosts your monthly checks by 8%. So it often pays to wait on Social Security, especially if you have reason to believe you'll live a longer life, such as being in great health and having a strong family history.

Those delayed retirement credits stop accruing at age 70, though. So if you're 70 already and you haven't signed up for Social Security yet, you should file for benefits immediately.

If you turned 70 a few months ago and haven't started benefits yet, don't panic. Social Security will generally pay up to six months of benefits retroactively, so you may not lose out on any income if you didn't sign up as soon as your 70th birthday arrived. But if that's the case, you certainly don't want to delay any longer, since it won't put any more money in your pocket.

You don't have to worry about working

You may have been holding off on claiming Social Security because you're still working. Even if you want to continue working, that's not something to worry about.

Social Security recipients are allowed to earn money from a job while collecting benefits. If you do so before reaching full retirement age, you'll be subject to an earnings test. And exceeding its limit could mean having benefits withheld.

However, at 70, you're well beyond full retirement age. This means you can earn any amount of money you want without a negative impact on your monthly Social Security checks.

That said, if you're collecting a boosted benefit along with a paycheck from a job, your total income may be high enough that you're subject to taxes on your Social Security payments. If you're on Medicare, a higher income could also result in surcharges on your premiums.

So if you're expecting your income to increase substantially after filing for Social Security, you may want to talk to a professional to discuss strategies for minimizing the tax hit.