Map<Integer, List<Person>> byAge = people.stream()
                .collect(Collectors.groupingBy(Person::getAge));

// Before Java 8
Map<Integer, Integer> ageQuantity = new HashMap<>();
for (Map.Entry<Integer, List<Person>> entry : byAge.entrySet()) {
    ageQuantity.put(entry.getKey(), entry.getValue().size());
}

// Java 8
Map<Integer, Integer> ageQuantity = new HashMap<>();
byAge.forEach((k, v) -> ageQuantity.put(k, v.size()));