She's only 23 or so though, isn't she? I would imagine she seems immature because she lived at home for uni and stuff. I would imagine that being in the modelling world she would have been exposed to drink and drugs. I just guess it depends on how we're defining immaturity. She seems very responsible, so mature in that sense, but I do think that she sometimes plays up the 'childlike' persona.
When I think back to when I was 23, I wasn't exactly mature, but I thought I was because I'd done uni, living away from my parents, taking care of rent and bills etc, but in other ways I was super immature. I read somewhere that the 30s are the new 20s and 20s are the new teens because of how society is at the moment. I think that's kind of reductivist, but I do kind of get it. I think Grace is just very fun loving. I think her attitude is actually quite mature for a 23 year old - saving for a house, thinking about a career etc.
Idk