function average(array) {
  function plus(a, b) { return a + b; }
  return array.reduce(plus) / array.length;
}

var byName = {};
ancestry.forEach(function(person) {
  byName[person.name] = person;
});

var hasMotherAncestry = ancestry.filter(function(person) {
    if(byName[person.mother] != null)
      return person;
  });

var arrayMomsAgeAtBirth = hasMotherAncestry.map(function(iterator) {
  return iterator.born - byName[iterator.mother].born
});

console.log(average(arrayMomsAgeAtBirth));