How about these solutions?

declare @DOB date = '20080229',

@CurDate date = '20130228';

select

@DOB as DateOfBirth,

@CurDate as CurrentDate,

datediff(yy, @DOB, @CurDate) - case when dateadd(yy, -datediff(yy, @DOB, @CurDate), @CurDate) < @DOB

then 1

else 0

end as TurnsYearOlderFirstOfMarch,

datediff(yy, @DOB, @CurDate) - case when dateadd(yy, datediff(yy, @DOB, @CurDate), @DOB) <= @CurDate

then 0

else 1

end as TurnsYearOlderLastOfFebruary;

set @CurDate = '20130301';

select

@DOB as DateOfBirth,

@CurDate as CurrentDate,

datediff(yy, @DOB, @CurDate) - case when dateadd(yy, -datediff(yy, @DOB, @CurDate), @CurDate) < @DOB

then 1

else 0

end as TurnsYearOlderFirstOfMarch,

datediff(yy, @DOB, @CurDate) - case when dateadd(yy, datediff(yy, @DOB, @CurDate), @DOB) <= @CurDate

then 0

else 1

end as TurnsYearOlderLastOfFebruary;

set @CurDate = '20130227';

select

@DOB as DateOfBirth,

@CurDate as CurrentDate,

datediff(yy, @DOB, @CurDate) - case when dateadd(yy, -datediff(yy, @DOB, @CurDate), @CurDate) < @DOB

then 1

else 0

end as TurnsYearOlderFirstOfMarch,

datediff(yy, @DOB, @CurDate) - case when dateadd(yy, datediff(yy, @DOB, @CurDate), @DOB) <= @CurDate

then 0

else 1

end as TurnsYearOlderLastOfFebruary;