Calculate Age in SQL Server using Query example

Hi! I have table population. I want count Age use this formula:

SELECT CASE CONVERT(INT,DATEDIFF(DD,BIRTHDAY,GETDATE())/365.25)WHEN 0 THEN 'till 1 year'
ELSE CONVERT(INT,DATEDIFF(DD,BIRTHDAY,GETDATE())/365.25) END AS AGE FROM KARTA

When Age = 0 then write 'till 1 year' else show year. I want use operator CASE. How i can solve this problem? Pleasse!!!