The answers given by chuff and teylyn work fine for scientific calculations; e.g., the normal height and weight of a person of a given age, or how long a bottle of wine has aged. This may be good enough for you. But they’re not perfect for legal purposes; e.g., on 5/31/2013, their formula computed 16.99931554. Admittedly, this will display as `17.00`

if displaying two decimal digits, but it will test as being less than 17. Not good if you are trying to compute somebody’s age in the normal, societal/legal sense, that we would use to determine, for example, whether he’s old enough to buy that bottle of wine.

For brevity, I’ll use the alternative suggestion that the ancient date is in `A1`

, and that today’s date is in `T1`

. I suggest that you use this formula:

```
=(YEAR(T1)-YEAR(A1)) + (DATE(YEAR(A1),MONTH(T1),DAY(T1))-A1)/365.25
```

This computes the difference between the two years, and then computes the fractional year difference between the start date and the current date *mirrored in the ancient year*. For example,
I’m writing this on June 12, 2013. The first part of the above formula, `YEAR(T1)-YEAR(A1)`

(the surrounding parentheses aren’t necessary) is `2013-1996`

which, of course, is exactly `17`

.
The second term,

```
(DATE(YEAR(A1),MONTH(T1),DAY(T1))-A1)/365.25
```

becomes `(DATE(1996,6,12)-A1)/365.25`

or `("6/12/1996"-"5/31/1996")/365.25`

or `12/365.25`

,
which is `0.03012`

, so the sum is `17.03012`

.
But if `T1`

is `5/31/2013`

, the formula evaluates to `17.00000`

.

Oh, my formula has a slight glitch. If the ancient date is in a non-leap-year (e.g., 5/31/1995), and the current year is a leap year (e.g., 2012), then `T1`

= `2/29/2012`

and `T1`

= `3/1/2012`

will yield the same result, because, even though Excel raises an error for `2/29/1995`

, it treats `DATE(1995,2,29)`

as `3/1/1995`

.

`TODAY()/365.25`

. This evaluates to (current date) - (epoch date), in years; i.e., 2013-1900, plus 0.41, since we’re almost halfway through 2013. See chuff’s answer for how to fix it. – Scott May 31 '13 at 0:02