I don't think that function should worry about time part of date of birth. In SQL2008 you better just to use DATE type so the time wouldn't be present. Pre-2008 I would enforce that DOB DATETIME column would only contain date part, so I wouldn't worry about time in queries which uses it.

Actually, I'm not even sure that UDF is really required here... I would probaley just use in-line cross apply, something like that:

-- here is a sample table with DOB as DATE only

DECLARE @MySample TABLE (DOB DATE)

INSERT @MySample

VALUES ('5 Nov 2012'),('3 Nov 2014'),('5 Nov 2011'),('6 Nov 2011'),('4 Apr 1973')

-- usually it would be single day and most likely it will be today...

DECLARE @datetoday DATE = GETDATE()

SELECT M.DOB

,AC.AGE

FROM @MySample M

CROSS APPLY (SELECT DATEDIFF(yy, M.DOB, @datetoday) -

CASE WHEN DOB > @datetoday THEN NULL

WHEN DATEADD(yy, DATEDIFF(yy, DOB, @datetoday), DOB) > @datetoday

THEN 1 ELSE 0

END AGE

) AC

Actually, if you want to calculate the AGE on the range of dates, you can use the following:

-- here is a sample table with DOB as DATE only

DECLARE @MySample TABLE (DOB DATE)

INSERT @MySample

VALUES ('5 Nov 2012'),('3 Nov 2014'),('5 Nov 2011'),('6 Nov 2011'),('4 Apr 1973')

-- Range of dates:

DECLARE @Dates TABLE (OnDay DATE)

INSERT @Dates

VALUES ('5 Nov 2012'),('10 Nov 2020'),('5 Nov 1970'),('5 Nov 2011'),('4 Apr 2073')

SELECT M.DOB

,D.OnDay

,AC.AGE

FROM @MySample M

CROSS JOIN @Dates D

CROSS APPLY (SELECT DATEDIFF(yy, M.DOB, D.OnDay) -

CASE WHEN DOB > D.OnDay THEN NULL

WHEN DATEADD(yy, DATEDIFF(yy, DOB, D.OnDay), DOB) > D.OnDay

THEN 1 ELSE 0

END AGE

) AC

ORDER BY M.DOB, D.OnDay