[tsql] How to calculate age in T-SQL with years, months, and days

I've seen the question several times with results outputting Years, Month, Days but never a numeric / decimal result. (At least not one that doesn't round incorrectly). I welcome feedback on this function. Might not still need a little adjusting.

-- Input to the function is two dates. -- Output is the numeric number of years between the two dates in Decimal(7,4) format. -- Output is always always a possitive number.

-- NOTE:Output does not handle if difference is greater than 999.9999

-- Logic is based on three steps. -- 1) Is the difference less than 1 year (0.5000, 0.3333, 0.6667, ect.) -- 2) Is the difference exactly a whole number of years (1,2,3, ect.)

-- 3) (Else)...The difference is years and some number of days. (1.5000, 2.3333, 7.6667, ect.)

CREATE Function [dbo].[F_Get_Actual_Age](@pi_date1 datetime,@pi_date2 datetime)
RETURNS Numeric(7,4)
AS
BEGIN

Declare 
 @l_tmp_date    DATETIME
,@l_days1       DECIMAL(9,6)
,@l_days2       DECIMAL(9,6)
,@l_result      DECIMAL(10,6)
,@l_years       DECIMAL(7,4)


  --Check to make sure there is a date for both inputs
  IF @pi_date1 IS NOT NULL and @pi_date2 IS NOT NULL  
  BEGIN

    IF @pi_date1 > @pi_date2 --Make sure the "older" date is in @pi_date1
      BEGIN
        SET @l_tmp_date = @pi_date2
        SET @pi_date2 = @Pi_date1
        SET @pi_date1 = @l_tmp_date
      END

    --Check #1 If date1 + 1 year is greater than date2, difference must be less than 1 year
    IF DATEADD(YYYY,1,@pi_date1) > @pi_date2  
      BEGIN
          --How many days between the two dates (numerator)
        SET @l_days1 = DATEDIFF(dd,@pi_date1, @pi_date2) 
          --subtract 1 year from date2 and calculate days bewteen it and date2
          --This is to get the denominator and accounts for leap year (365 or 366 days)
        SET @l_days2 = DATEDIFF(dd,dateadd(yyyy,-1,@pi_date2),@pi_date2) 
        SET @l_years = @l_days1 / @l_days2 -- Do the math
      END
    ELSE
      --Check #2  Are the dates an exact number of years apart.
      --Calculate years bewteen date1 and date2, then add the years to date1, compare dates to see if exactly the same.
      IF DATEADD(YYYY,DATEDIFF(YYYY,@pi_date1,@pi_date2),@pi_date1) = @pi_date2  
        SET @l_years = DATEDIFF(YYYY,@pi_date1, @pi_date2) --AS Years, 'Exactly even Years' AS Msg
      ELSE
      BEGIN
        --Check #3 The rest of the cases.
        --Check if datediff, returning years, over or under states the years difference
        SET @l_years = DATEDIFF(YYYY,@pi_date1, @pi_date2)
        IF DATEADD(YYYY,@l_years,@pi_date1) > @pi_date2
          SET @l_years = @l_years -1
          --use basicly same logic as in check #1  
        SET @l_days1 = DATEDIFF(dd,DATEADD(YYYY,@l_years,@pi_date1), @pi_date2) 
        SET @l_days2 = DATEDIFF(dd,dateadd(yyyy,-1,@pi_date2),@pi_date2) 
        SET @l_years = @l_years + @l_days1 / @l_days2
        --SELECT @l_years AS Years, 'Years Plus' AS Msg
      END
  END
  ELSE
    SET @l_years = 0  --If either date was null

RETURN @l_Years  --Return the result as decimal(7,4)
END  

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