Question: Calculate the difference in the average age of elderly and middle-aged clients in the Northeast region.
SQL: SELECT (CAST(SUM(CASE WHEN T1.age BETWEEN 35 AND 55 THEN T1.age ELSE 0 END) AS REAL) / SUM(CASE WHEN T1.age BETWEEN 35 AND 55 THEN 1 ELSE 0 END)) - (CAST(SUM(CASE WHEN T1.age > 65 THEN T1.age ELSE 0 END) AS REAL) / SUM(CASE WHEN T1.age > 65 THEN 1 ELSE 0 END)) AS difference FROM client AS T1 INNER JOIN district AS T2 ON T1.district_id = T2.district_id INNER JOIN state AS T3 ON T2.state_abbrev = T3.StateCode WHERE T3.Region = 'Northeast'