PREFIX foaf: <http://xmlns.com/foaf/0.1/>
PREFIX data: <http://example.org/foaf/>

SELECT ?src ?bobAge
GRAPH   data:aliceFoaf data:bobFoaf
WHERE
  SOURCE ?src
  {
    ( ?x foaf:mbox <mailto:bob@work.example> )
    ( ?x foaf:age ?bobAge )
  }