I have no problem with the characters being younger, I have a problem with atrocious and stupid dialogue which has become evident in all three issues of Justice League so far. For instance upon Wonder Woman's first foray into battle as Superman is witnessing her he notes "You're strong." Really? Yeah there was just bad dialogue all through the book.
Like I say, I haven't read the third one. I thought the cell phone conversation between Flash and GL in the second issue was pretty entertaining.
It's possible I have a higher threshold for bad dialogue from young people because I'm a teacher, and many of my students (quite capable at times) tend to speak in a combination of heightened rhetoric, catch phrases, and absolutes. So a young Hal Jordan calling dibs looks much like football players I've overheard at the pub. It rings a little bit true for me.
Then again, maybe Johns is just phoning it in and I'm making excuses for him!