Rangi aged up a few times, the reason she reverts is that he will have to marry her if she stays in the adult body that makes sense as her body is a reflection of their mental age (mentality) and thats kinda of a line he will not cross even if she keeps pestering him about it.
And this is the difference of a loli and a child, its kinda amusing since this is mentioned more that a few times (especially in relation to Rangi) that Phantoms body are more of a reflection of their mentality that actual age ...