Question: What percentage of clients who sent their complaints by postal mail are age 50 and older?
SQL: SELECT CAST(SUM(CASE WHEN T1.age > 50 THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(T2.`Submitted via`) FROM client AS T1 INNER JOIN events AS T2 ON T1.client_id = T2.Client_ID WHERE T2.`Submitted via` = 'Postal mail'