I'm not saying it's ideal and I would agree that it should be a transitional phase. I simply object to the implication that adults who live with parents are psychological children (emotionally or intellectually). There are really many different reasons why an adult might be in this situation.
I know one man who lives with his grandparents (parents are dead). He is a registered sex-offender due to something that he allegedly did when he was 13. As a result most employers won't even consider hiring him and, even if he could get a job, most apartment managers cannot legally rent to him.
I myself have attempted to get an apartment several times but because I have no rental or credit history I was turned down repeatedly, even though I made more than enough income to afford the monthly rent and offered to pay them three months advance rent if they gave me chance.
My point is that just because some people are not able to house themselves does not necessarily make them immature or irresponsible. They might just be poor, or physically disabled, or have a criminal background, or simply lack the social skills needed to make it in the world. None of these disadvantages makes them children.