SELECT FirstName, LastName,
    CAST(
        CASE
            WHEN FLOOR((CAST (GetDate() AS INTEGER) - CAST(dob AS INTEGER)) / 365.25) < 18
                THEN 'Under 18'
            WHEN FLOOR((CAST (GetDate() AS INTEGER) - CAST(dob AS INTEGER)) / 365.25) BETWEEN 19 AND 30
                THEN '19-30'
            WHEN FLOOR((CAST (GetDate() AS INTEGER) - CAST(dob AS INTEGER)) / 365.25) > 30
                THEN 'Over 30'
            ELSE 'ERROR'
    END AS VARCHAR(10)) AS Age
FROM Student

SELECT FirstName, LastName,
       (CASE WHEN v.age <= 18 THEN 'Under 18'
             WHEN v.age <= 30 THEN '19-30'
             WHEN v.age > 30 THEN 'Over 30'
             ELSE 'ERROR'
        END) AS Age
FROM Student s OUTER APPLY
     (VALUES (FLOOR((CAST (GetDate() AS INTEGER) - CAST(dob AS INTEGER)) / 365.25) )
     ) as v(age);