I'm fine with constructing character personalities; but making the dialogue sound unique to each one is where I have the most significant trouble.
Obviously, not all teenagers are the same, but I feel that generally speaking there are some majors differences. From my own (admittedly very limited experience) I've noticed that children tend to be more straightforward in their speech. What I mean is, they don't usually hide their meanings in subtlety, innuendo, metaphor, or making allusions to other things such as "give to Caesar what is Caesar's. . .", which unfortunately is the sort of thing I love to do when writing dialogue. It's just such a great way to combine worldbuilding with character backstory, but it doesn't feel natural for younger characters and this is a major difficulty I have.
"You're a bully, Johnathan," I snapped. I've known him for years, but not this side of him. I wanted to yank that scab off his chin.