Smidsy have you worked this out yet? This is how I would do it:

Not sure why you need an =if argument mate? - Only if you wanna not show a zero value if nothing is in your input cell

If you make A1 (substitute $A$1 as necessary or use tools, name, define, and name that sngle input cell) the input cell you put your date of birth in the the, and put the forumla elsewhere, it should be:

=round(sum((today()-$A$1)/365),0)

or to keep answer cell blank until something is in the input cell:

=if($A$1="","",round(sum((today()-$A$1)/365),0))

$A$1 is now an absolute reference and wherever you copy this formula to on the worksheet will still refer to this cell. Use right mouse button and format then date to get how you want the date to appear.

BTW Excel looks at dates by number of days from the date 1st Jan 1900, (on apple macs the date is 1904 for some reason). So if you only have a general format in place and type in todays date it will be something like 38003 I.e. thats how many days from 1/1/1900 it is. Doesn't matter how you format the cell, Excel still looks at the date in number of days - its just how you view it that changes.

This should work - but a haven't got excel at home so can't check. Have fun mate