PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT ?mbox ?age ?ppd
WHERE       ( ?alice foaf:mbox <mailto:alice@work.example> )
            ( ?alice foaf:knows ?whom )
            ( ?whom foaf:mbox ?mbox )
            ( ?whom foaf:PersonalProfileDocument ?ppd )
SOURCE ?ppd ( ?whom foaf:age ?age )