Look at SimpleDateFormat. You can use that to parse a
String into a Date object.
If you build Date objects for current time and birth date, you can work out the time between them with pretty simple math. Look at the
doc for Date ... do you see a method that returns a number we might do math on? See if you can compute age in days or years. Avoid months - they're all different sizes which makes them mostly meaningless.
These are pretty vague hints ... I hope you have fun digging out the details yourself.