declare @dob datetime,
        @age int,
        @day datetime

set @day = '2008-12-15'
set @dob = '2007-02-28'
set @age = datediff(yy,@dob,@day) -
           case when @day < dateadd(yy, datediff(yy,@dob,@day), @dob) then 1 else 0 end

select
    case
        when @day < dateadd(mm, 1, @dob)
            then cast(datediff(dd, @dob, @day) as varchar) + 'd'
        when @day < dateadd(mm, 3, @dob)
            then cast(datediff(wk, @dob, @day) as varchar) + 'w'
        when @day <= dateadd(yy, 1, @dob)
            then cast(datediff(mm, @dob, @day) as varchar) + 'm'
        else cast(datediff(yy,@dob,@day) -
                  case when @day < dateadd(yy, datediff(yy,@dob,@day), @dob) then 1 else 0 end as varchar) + 'y'
    end