query: # average age of members of the 19th German Bundestag 
  SELECT (AVG(?age) AS ?avgAge) WHERE {
    ?mdb wdt:P31 wd:Q5;
         p:P39 [
           ps:P39 wd:Q1939555;
           pq:P2937 wd:Q30579723
         ];
         wdt:P569 ?dob.
    BIND(FLOOR((NOW() - ?dob)/365.2425) AS ?age)
  }