The mangaka has no idea what a five years old little squirt looks like. A five years old is still a year or two away, depending on where you live, even from starting the bloody elementary school. But then again, the young Ars's design has been very varying from the beginning. In the first chapter he looked like anything from maybe 8 to 15, depending on the panel. Let's also not forget that he had lived a malnourished early life, so he should look younger (smaller) than his real age would suggest.
Well, the art is otherwise good, so I shouldn't complain that much.