declare @birthday datetime, @d datetime
select @birthday = '12/31/1998', @d = '1/1/99'
select datediff(yy, @birthday, @d) -
	(case when (datepart(m, @birthday) > datepart(m, @d)) or
			(datepart(m, @birthday) = datepart(m, @d) And
				datepart(d, @birthday) > datepart(d, @d))
			then  1 
			else  0 
	end) as Age1
 -- Just for completion, another way to do the same
 
select year ( dateadd( dd, datediff ( dd , @birthday , @d ) ,
		'1900-01-01 00:00' ) )	-  1900 
 -- To find out if a year is leap year or not, use
 
if ( year ( @d ) %  400  =  0  ) Or
	( year( @d ) %  4  =  0  And year ( @d ) %  100  <>  0  )
	print 'Leap Year - 366 days'
else
	print 'Not a leap year - 365.25 days'