Competition of u.s championships, and a Event of all around has what average year?
CREATE TABLE "2006_season_junior_career" (
  "year" real,
  "competition" text,
  "location" text,
  "event" text,
  "final_rank" real,
  "final_score" real
);
SELECT AVG("year") FROM "2006_season_junior_career" WHERE "competition"='u.s championships' AND "event"='all around';