I intuitively choose 2 because 1 and 3 are definitely wrong. 1 doesn't take care of month and day of birth at all and 3 does it in the wrong way. But the are all wrong. It can be easily verified with the script below. Play around with some values at the beginning and end of the month in both dates. I added a fourth solution which I think is correct.

declare @DateOfBirth datetime

declare @Today datetime

select @DateOfBirth = '2004/03/01'

select @Today = '2008/02/29'

select

DATEDIFF(yy, @DateOfBirth, @today)

select

FLOOR(CONVERT(decimal(9, 2), DATEDIFF(d, @DateOfBirth, @Today)) / 365.0)

select

DATEDIFF(yy, @DateOfBirth, @today)

- CASE

WHEN DATEPART(m, @DateOfBirth) >= DATEPART(m, @Today) AND DATEPART(d, @DateOfBirth) >= DATEPART(d, @Today) THEN

0

ELSE

1

END

select

DATEDIFF(yy, @DateOfBirth, @today)

- CASE

WHEN DATEPART(m, @Today) > DATEPART(m, @DateOfBirth)

OR (DATEPART(m, @Today) = DATEPART(m, @DateOfBirth)

AND DATEPART(d, @Today) >= DATEPART(d, @DateOfBirth)) THEN

0

ELSE

1

END