CREATE TABLE table_204_26 (id DECIMAL, "name" TEXT, "position" TEXT, "birth date/age at start of campaign" TEXT, "games in campaign" TEXT, "club" TEXT)

what was the average age of the grasshoppers ?
SELECT AVG("birth date/age at start of campaign") FROM table_204_26 WHERE "club" = 'grasshoppers'
SELECT AVG("birth date/age at start of campaign") FROM "table_204_26" WHERE "club" = 'grasshoppers'