This is because DateDiff only looks at the interval you specify. In this case, whole years. It doesn't take the months and days into consideration. So whether you enter:

DateDiff("yyyy",#10/10/1981#,#10/20/2020#)

or

DateDiff("yyyy",#10/30/1981#,#10/20/2020#)

you're going to get 39 because DateDiff is only calculating 2020-1981.

A better "simple" way to calculate age is like this:

Int((Date()-[DOB])/365.24)

So if you enter

Int((#10/20/2020#-#10/10/1981#)/365.24)

you get 39, but if you enter

Int((#10/1/2020#-#10/10/1981#)/365.24)

you get 38 because the person's birthday has not passed yet. This is a "good enough" example and it's correct 99% of the time, but it's not perfect. Leap years can throw it off. I cover a PERFECT way to calculate age in Access Expert 28.

BTW, in your example, the correct age SHOULD be 39. If someone's birthday is 10/10 and today is 10/20 then their birthday has passed, therefore 39 is the correct answer. But I understand your question.

This is an excellent question. Again, I do cover it in X28, but I'm going to add this to my TechHelp list for a future short video.

New TechHelp: Calculate Age