One of the things that Excel's DATEDIF function can do is to calculate a person's age in years, months, and days.
It can do this because DATEDIF allows you to change the interval between two dates from years to months or days.
One thing however, DATEDIF is undocumented - which means you won't find it listed with other functions on the Formulas tab of the ribbon in Excel.
It also means that the whole formula must be typed in by hand into a worksheet.
To find out the details of using this function, read this tutorial on Calculate Your Current Age with DATEDIF.
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