For most Americans, Social Security is the single largest source of retirement income — and the decision of when to start collecting it is effectively permanent. Claim early and you lock in a smaller check for life. Wait, and every additional month of patience is quietly rewarded.

Yet nearly two-thirds of retirees claim before their full retirement age, often at 62, the earliest possible moment. Some have no choice. Many simply underestimate how much that early start will cost them across a retirement that could span three decades.

How the math actually works

Your full retirement age, depending on your birth year, falls between 66 and 67. Claim at 62 and your benefit is reduced by as much as 30 percent — for good. Delay past full retirement age and you earn delayed retirement credits worth roughly 8 percent a year until you turn 70.

That 8 percent is not a market return you have to hope for. It is a guaranteed, inflation-adjusted increase written into the program itself, which is why advisers so often describe delaying as the best deal in retirement finance.

The questions that should drive your decision

• How is your health, and what does longevity look like in your family?
• Are you still working, and would early benefits be reduced by the earnings test?
• Is there a spouse whose survivor benefit depends on your record?
• Do you have other assets that can bridge the years before you claim?

There is no universally correct age. A single person in poor health may be entirely right to claim early, while a healthy higher earner with a younger spouse often has the most to gain from waiting. The goal is to make the choice deliberately, with the full picture in front of you.