Professor T. Ripaldi
I teach 2nd grade. People often talk about "childish behavior". For example, I have students who care very deeply about their position in line. They get aggrieved if folks "butt" them in line yet shrug it off when they do it to someone else. Sometimes it gets so intense, tears or fists come out. Although this kind of behavior can be broad-brushed to all children, only about a third behave this way. Most of my students couldn't care less.
On the other end of the spectrum. You will often hear people assume all "old folks" think in similar ways. The comedian Bill Burr does a whole routine about this. He questions why the family was surprised when Grandma said racist things.
"They're old! he jokes. "What didja think they thought?"
People often use old age to shrug off racism or homophobia... as if it is just part of being old. Yet, we all know plenty of old folks who don't hold these attitudes.
I think, as the professor says above, people already are. Little Johnny may move on from caring about who stands where in line, but that core pettiness will probably just transfer to other things. Later in life, he may care deeply that he has a better car or house than the person behind him.
Behaviors are not age-related. One of my students can nurse a grudge for weeks... while another has let an offense go by recess. They will most likely become adults who carry those same traits.
The thing is, if we become an old person that is STILL racist or STILL petty or STILL holds grudges, etc... then life passed us by and we didn't do the real work that needed to be done.