Counting persons in each age range
Java 8 streams

Aside from the typos pointed out by @SuperBiasedMan, you can use Java 8's stream-based processing as an alternative for generating your Map result.

private static final int UPPER_LIMIT = 100;
private static final String[] KEYS = new String[]{ "00 - 09", /* ... */ "Unknown" };

private static int categorize(int year) {
    return Math.min(year, UPPER_LIMIT) / 10;
}

Map&lt;String, Long&gt; results = persons.stream()
                    .map(Person::getDob)
                    .filter(Objects::nonNull)
                    .map(ThisClass::getDiffYears)
                    .map(ThisClass::categorize)
                    .collect(Collectors.groupingBy(ageCategory -&gt; KEYS[ageCategory],
                                Collectors.counting()));



Define the UPPER_LIMIT of your age categorization and the desired category names.
From a Stream of your Person objects, map() to their birthdays by calling Person::getDob as method reference.
filter() null birthdays by using Objects::nonNull.
map() birthdays to ages by calling your getDiffYears(Date) method. Note that if it is not a static method, the method reference then becomes this::getDiffYears.
map() ages to the age category by calling categorize(int) to get the desired array indices for KEYS. If you have not realized yet, ThisClass is just a placeholder for the actual class name.
collect() the age categories into the desired Map result groupingBy() our categorization and counting() the elements per category.


Other advice


Your method should return a Map instead of HashMap so that callers of this method only need to deal with the interface, and not the implementation.
You may want to consider how to handle for null inputs to getDiffYear(Date). Should it return a negative value?
You can also experiment with deriving the category names in a programmatic manner, instead of using literal Strings.
You can even think about making your age categorization depend on a configurable upper limit (100) and band (10). For example, how easy will it be to treat the upper limit as 101 (UPPER_LIMIT = 101) and a band of 11 years per category (hint: take a closer look at the suggested code...)?