Normal retirement age is set in the plan document. It is the age at which the benefit formula is measured and the benefit becomes fully payable. IRC 411(a)(8) caps it at the later of 65 and the fifth anniversary of joining the plan. Treasury regulations accept 62 or older in every case; an age from 55 to 62 depends on the facts, and under 55 is presumed too early (Treas. Reg. 1.401(a)-1(b)(2)).
One-person plans commonly choose 62, for two reasons. The 415(b) dollar limit is available in full from 62 and reduced before it. And the factor that turns that yearly limit into a lump sum is larger at 62 than at 65, so the lifetime maximum is highest at 62.
Contributions are spread over the years left before retirement, so the fewer years remain, the more each year's contribution can be. For owners already close to 62, the actuary assumes a retirement date some years out. Maxed's estimates use the later of age 62 and 5 years after the plan starts, a conservative convention.
Normal retirement age does not require you to stop working. It is the age the plan's figures are built around. See how much you can contribute.