declare @dob as datetime
select @dob = '6/21/1968'
declare @now as datetime
declare @year_diff as tinyint
select @now = getdate()
select @year_diff = datediff(year, @dob, @now)
if DateAdd(year, @year_diff, @dob) > @now
	select @year_diff -1
else
	select @year_diff