Once you have a Calendar object initialized with the correct date, you can call the get(Calendar.YEAR) value to get an integer representing the year. Do this for both dates and subtract.
You'll also want to get the month and date and compare, because if the end date has a lower month/date than the start, then you'll want to subtract 1 from the earlier result (the year has not yet been completed).
Typically for birthdays you don't care about the time component (hour/minute/second), so some libraries may give you a different result (in an extremely small number of cases) -- it's justifiable to roll your own for this case.
