They probably did it for the comedy/weirdness angle they're going with. Which is sad because Skarsgaard does really good voices when he gets into a role.
Also looking over this again and wondering because of how it's edited. Is it just the "boy" that's grown up and not the girl? Like, are they doing something like where it's similar to OPM and the kid trains to where he's all adult physically but still a kid mentally? Slightly confused on that.