Conditionals On Derived Columns

Hi,

Here's my current query, which throws an error that "AgeCalc" is an invalid column in the WHERE clause:

---------------------------------
SELECT
.
.
.,
AgeCalc =
CASE
WHEN dateadd(year, datediff (year, B.DOB, B.DateIn), B.DOB) > B.DateIn
THEN datediff (year, B.DOB, B.DateIn) - 1
ELSE datediff (year, B.DOB, B.DateIn)
END

FROM
ResidentData B

WHERE
(AgeCalc >= 18)
---------------------------------

How do I do conditionals on the "AgeCalc" derived column?

Thanks.