Problem solving

Recently a mate of mine had been mulling over a puzzle that he’d found via Reddit and couldn’t solve. It goes something like this “You can only be exactly twice someone’s age once, and it’s when they turn the same age you were when they were born. Every birthday after that makes you less than twice their age”. So I thought, ‘Hang on a moment! I’ve been writing calendar orientated small Java programs, Java has a “GregorianCalendar” class that I can use, and you can perform time functions! Challenge accepted!’.