Lower tax from AOW age
From your state pension (AOW) age you no longer pay the AOW premium, so the first-bracket rate drops sharply. You also get the elderly tax credit (ouderenkorting), and the general tax credit is lower. The calculators apply this when you tick the AOW option.
How pensions are taxed
AOW and workplace pensions are taxed as income in Box 1. The SVB and your pension fund withhold payroll tax. Only one of them should apply the tax credits (loonheffingskorting), usually the one paying most, or you may have to pay back tax later.
An AOW gap if you lived abroad
You build up 2% of the full AOW for each year you lived or worked in the Netherlands in the 50 years before your AOW age. Years abroad leave a gap. You can sometimes insure the gap voluntarily, and a pension from another EU country or a treaty country may cover those years. See pensions and saving for retirement.
Sources
This article is based on these official pages. If anything here differs from them, the official page is right.
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