What is the date of birth for the player from Ulster and plays at Centre position?
CREATE TABLE "2008_six_nations_championship_squads" (
  "player" text,
  "position" text,
  "date_of_birth_age" text,
  "caps" real,
  "club_province" text
);
SELECT "date_of_birth_age" FROM "2008_six_nations_championship_squads" WHERE "club_province"='ulster' AND "position"='centre';