Don't worry all sorted:

Declare @DOB Datetime

Declare @Today Datetime

Set @DOB = '01 November 1971'

Set @Today = '01 November 2011'

SELECT

CASE WHEN DatePart(Month, @Today) < Datepart(Month,@DOB) THEN (Datediff(Year,@DOB,@Today) - 1)

WHEN DatePart(Month, @Today) > Datepart(Month,@DOB) THEN Datediff(Year,@DOB,@Today)

WHEN DatePart(Month, @Today) = Datepart(Month,@DOB) THEN

CASE

WHEN DatePart(Day, @Today) < Datepart(Day,@DOB) THEN (Datediff(Year,@DOB,@Today) - 1 )

WHEN DatePart(Day, @Today) >= Datepart(Day,@DOB) THEN (Datediff(Year,@DOB,@Today) )

END

END AS How_Old_Am_I