This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
You may convert the birthday in second unit(note that before 1970 is negative value). Then get the current time using System.getCurrentTimeMillis() and change it to second by dividing 1000. Define number of second in a year which is 365*24*60*60. Then age should be (current_time-birthday)/second_year. Just a thought.