What is the date settled for 40 years?
CREATE TABLE "suburbs" (
  "suburb" text,
  "population_in_2008" real,
  "median_age_in_2006" text,
  "mean_household_size_in_2006" text,
  "area_km" text,
  "density_km" real,
  "date_first_settled_as_a_suburb" real,
  "gazetted_as_a_division_name" text
);
SELECT "date_first_settled_as_a_suburb" FROM "suburbs" WHERE "median_age_in_2006"='40 years';