Here's a slightly different version:

DECLARE @DOB DATE = '20000101';

DECLARE @AgeYears TINYINT = DATEDIFF(YEAR, @DOB, GETDATE());

SELECT Age = (CASE WHEN @AgeYears <= 13 THEN 'Age less than 14'

WHEN @AgeYears BETWEEN 14 AND 15 THEN 'Age 14 to 15'

WHEN @AgeYears BETWEEN 16 AND 17 THEN 'Age 16 to 17'

WHEN @AgeYears = 18 THEN 'Age 18'

WHEN @AgeYears BETWEEN 19 AND 20 THEN 'Age 19 to 20'

WHEN @AgeYears BETWEEN 21 AND 25 THEN 'Age 21 to 25'

WHEN @AgeYears BETWEEN 26 AND 35 THEN 'Age 26 to 35'

WHEN @AgeYears BETWEEN 36 AND 45 THEN 'Age 36 to 45'

WHEN @AgeYears BETWEEN 46 AND 55 THEN 'Age 46 to 55'

WHEN @AgeYears BETWEEN 56 AND 65 THEN 'Age 56 to 65'

WHEN @AgeYears >= 66 THEN 'Age 66+'

END);