Adulting is hard sometimes. No one can behave like a grown-up from dawn to dusk, and we all slack off now and then. Unfortunately for your fictional people, they don't get to slack off when they feel like it, because perception is everything.

As a result—and this is one of those things I always emphasize—it isn't enough to be right. Your reader has to believe that you are right.

A recent example from my work in progress involves an introductory scene for the main character that featured a conversation between her and her father. She responded entirely in character, but she also responded with a certain degree of childishness. You know how it is: you're talking to someone who held you in the crook of his arm in a rocking chair, who put the star on top of the Christmas tree because only he could reach, and who used to shield you from every bad thing in life. Under those circumstances, it's only natural to feel about an inch tall. In fact, those moments when our fathers aren't larger-than-life can be the most disturbing moments in our lives.

...Even so, it was not effective for me to portray my protagonist in that way. Accurate, yes. But not effective.

I went back and reworked that scene so that she screened his call and he sent a text afterward. She was able to give her reaction (directly to the reader) to what he said, but her reaction to actually talking to him was curtailed. That allowed me to avoid portraying the kind of childish response she has to him, because she is still awed by him in many ways.

Hopefully she seems 20 now instead of 12. :)