Technically, while light-years are a measurement of length, one can also argue that it can be used as a measurement of time in that it takes exactly one year for light to travel the length. So perhaps Rantz is measuring his age by measuring the distance (in light-years) that light has travelled in a vacuum since his birth... which is pretty much exactly how the rest of us measure our age, too.

Alternatively, since he didn't include the hyphen, perhaps he is measuring his age in years that he considers to lack in weight. How he weighs a year is a mystery.