Both of the guys are born on the same day, month, and year but they have different ages. One of them is 5 months, 12 days, and 11 years old and the other is 2 months, 17 days, and 11 years old.
Why are there two different ages then?
They were born at the same time.
There is no third child.
They were born on the same time and place.
They might not be on the same page here...
Now that people have guessed the idea of the answer, it is one of the most popular calendars in the world. (See comments for info)