This movie is not about arrested development and childish behavior
I had to write this post just to disagree with the pseudo-intellectuals who want to see in this movie a concretization of hoax pop-psychology theories.
First of all I don't think making a game out of sex is a childish thing.
Sex is also supposed to be a game, to be a fun way to create complicity between two or more people. Even chimps make games out of sex, so far for the idea that animals use sex just for reproduction.
Sex doesn't have to be a serious, haughty and full of faux responsability thing.
In fact I would worry more about the psychological state of someone who sees sex like a serious task to load with an heavy of psycho-bubble burden than of someone who can see the playful nature of sex.
Also I don't think the twins suffered from childish immaturity and were avoiding adult responsabilities. When people talk about "adult responsabilities" they depict a world where young people are carefree and have nothing to worry about and were older people don't have free time, don't know how to have fun and have huge responsabilities that weigh extremely on their shoulders.
Well, this world don't exist. For the majority of adults the extent of their responsabilities end with having a job and paying a bill. Certainly nothing to lose your sanity for. In fact most adults have a lot fun, take everything superficially, shut themselves in their family middle-class world while letting their intellectual capacity fade as muscles that are not worked-out and actually consider the tame task of working daily and paying the bill as something requiring even less responsability and effort than the responsabilities of teens and school. Because let's face it, the majority of jobs people do are not something that can be called "a huge responsability", they are even more of a game than the games small children game, they're actually easy, easier than school for sure. So I really don't know where this dichotomy between young people enthusiasm for life and adults serious and hard life come from. The young people part is true, it's the adults one that is a lie and something that makes them appear like hypocrite in the eyes of younger people.
So I hope people will stop talking about mature young people not "growing up and accepting adult responsabilities" just because they don't accept to conform to adult fake self-importance and seriousness. And would stop using the meaningless word "adult responsabilities" seeing as for more people their adult life is actually more responsabilities-free than any other stage of their life.
In fact for more "adult life" is a perfect way to choose a life of conformity, mental dullness and alienated life withing home and work-place with no need to expand one's mind, confront with the world or accept new responsabilities.