Maturity is a fickle subject, since no one cam truly define it.
When he says it's because of childish behaviour, it hints most likely to infantilization of people on basis of a set of acts or norms making up "childish". Truly, most of these aesthetic traits are not acclaimed by childhood.
When saying someone who has a trait not exclusive to childhood (which almost all are) is childish, thats most likely ableist.
When someone has big emotional expression, or dress "like a child".
But if they reffered to an authors choice to make an aged up but small loli, its therein a valid fear...