Why does Charley say Willy hasn't grown up in Death of a Salesman?

In Act II of
's  we find Willy Loman and his friend and neighbor, Charley talking to
each other during one of Willy's hallucinatory memories. 

During this
particular flashback, Willy is getting ready for Biff's biggest game in Ebbet's Field. We know
how Willy lives vicariously through Biff's personal accomplishments in football. To an average
person, this game should be about seeing Biff accomplish the goal of playing in a big field. To
Willy, judging from his extremely agitated mannerisms, we can tell that this event signifies the
beginning of Willy's own American Dream.

However, in the flashback, Willy's
high spirits are being subdued by Charley's composure. This makes Willy very mad, since in
reality Willy is absolutely secure in the idea that this night would be legendary for Biff, and
that he will become a famous football player from that moment on. 

This is
the reason why Charley, in view of Willy's ridiculous, and almost out of control behavior,
finally asks Willy 

Willy, when are you going to grow
up?

To which Willy answers: 

Yeah? Heh? When this game is over, Charley, you'll be laughing out of the other side of
your face. They'll be calling him another Red Grange. Twenty five thousand a year. 

Charley: (Kidding) Is that so?

Willy: Yeah that's so

Hence, we can see how Charley downplays Willy's behavior and
basically laughs at Willy's immature behavior practically to his face. Willy, far from
recognizing himself and his behavior, continues the childish talk with his annoying, almost
bratty and catty behavior toward Charley. It really makes us agree with Charley and question
ourselves as to when Willy will grow up. We realize, until the end, that Willy really never
does.