What is the average age at appointment of those attached to security?
CREATE TABLE "political_assistants" (
  "romanised_name" text,
  "chinese_name" text,
  "age_at_appointment" real,
  "foreign_nationality" text,
  "portfolio_attachment" text,
  "govt_salary" text
);
SELECT AVG("age_at_appointment") FROM "political_assistants" WHERE "portfolio_attachment"='security';