IF OBJECT_ID('tempdb..#AlephTest') IS NOT NULL
	DROP TABLE #AlephTest

SELECT
	[location_id] = 1
	, [customer_id] = 12345
	, [age] = 10
INTO #AlephTest
UNION ALL SELECT 1, 21752, 28
UNION ALL SELECT 1, 98712, 25
UNION ALL SELECT 1, 12896, 23
UNION ALL SELECT 2, 12874, 38
UNION ALL SELECT 2, 23987, 29
UNION ALL SELECT 2, 23876, 23
UNION ALL SELECT 2, 54961, 24

SELECT
	a.location_id
	, [age_group] = CASE 
				WHEN a.age < 18			THEN '<18'
				WHEN a.age BETWEEN 18 AND 35	THEN '18 to 35'
				WHEN a.age > 35			THEN '>35'
			END
	, [customer_count] = Count(*)
FROM #AlephTest a
GROUP BY
	a.location_id
	, CASE 
		WHEN a.age < 18			THEN '<18'
		WHEN a.age BETWEEN 18 AND 35	THEN '18 to 35'
		WHEN a.age > 35			THEN '>35'
	  END
ORDER BY
	a.location_id
	, CASE (CASE 
			WHEN a.age < 18			THEN '<18'
			WHEN a.age BETWEEN 18 AND 35	THEN '18 to 35'
			WHEN a.age > 35			THEN '>35'
		END)
		WHEN '<18'	THEN 1
		WHEN '18 to 35'	THEN 2
		ELSE 3
	  END

CREATE TABLE #AgeRange
(
RangeDesc varchar(20),
AgeStart int,
AgeEnd
)

INSERT #AgeRange
SELECT '<18',1,17 UNION ALL
SELECT '18 to 35',18,35 UNION ALL
SELECT '>35',36,120 

then use it like

SELECT
 m.location_id,m.RagenDesc,
COUNT(n.customer_id) AS Count
FROM
(
SELECT t.location_id,r.RangeDesc,r.AgeStart,r.AgeEnd
FROM (SELECT DISTINCT location_id FROM #Alephtest) t
CROSS JOIN #AgeRange r
)m
LEFT JOIN #Alephtest n
ON n.location_id = m.location_id
AND n.age BETWEEN m.AgeStart AND m.AgeEnd
GROUP BY m.location_id,m.RangeDesc