bruto
Penultimate Amazing
I find this kind of riddle very annoying, because it uses certain conventions and then pretends that it did not.
Once you have spoken of a persons age as "four years old," I think you are committed to speaking in the convention of "years old," in which a person is a certain age in years between one birthday and the next. In fact, at least in this culture, we tend to ridicule children or to consider them cutely childish, if they slice it any finer. When a child says "I'm four years and three months old," the statement is almost always belittled.
Thus, as far as I can see, the incompleteness consists merely of the fact that staggered birthdays can result in a variation for a period of time. If my sister turned three after I turned four, there is a period during which she was half my age, or two, and a period during which she was three, and during some of her age in years, I will be one year older, not two. If she turned two after I turned four, there will be a period during which I was four and she was one, as well as a period during which I was four and she was two, and during some of her age, I will be three years older, not two.
But if you don't mean the puzzle to use the standard meaning of "years old," then you should not begin with it.
Once you have spoken of a persons age as "four years old," I think you are committed to speaking in the convention of "years old," in which a person is a certain age in years between one birthday and the next. In fact, at least in this culture, we tend to ridicule children or to consider them cutely childish, if they slice it any finer. When a child says "I'm four years and three months old," the statement is almost always belittled.
Thus, as far as I can see, the incompleteness consists merely of the fact that staggered birthdays can result in a variation for a period of time. If my sister turned three after I turned four, there is a period during which she was half my age, or two, and a period during which she was three, and during some of her age in years, I will be one year older, not two. If she turned two after I turned four, there will be a period during which I was four and she was one, as well as a period during which I was four and she was two, and during some of her age, I will be three years older, not two.
But if you don't mean the puzzle to use the standard meaning of "years old," then you should not begin with it.