One more into the ring...

SELECT
CASE
WHEN DATEADD(year, DATEDIFF (year, @bdate, GETDATE()), @bdate) > GETDATE() THEN DATEDIFF (year, bdate, GETDATE()) - 1
ELSE DATEDIFF (year, bdate, GETDATE())
END

BTW, this comes from the SQL Server FAQ.

-Mike