I'll agree with lesfeuxetc. & Kit and disagree with the sharp one. I'm utterly annoyed by contractions and "phonetic" spelling. Trying to characterize someone with and accent or else doesn't work for me. The example coming to my mind is Hagrid in Harry Potter. They're all Brits (or almost) in the series, but he's the only one with this annoying accent. When he has too much dialogue, it's REALLY annoying.
As for teen, I got the same criticism from my friend/editor Nicholas H. about the characters in my Boarding School story. No way they're gonna say "gonna". I'm not too good at dialogues in general, and when not in my mother tongue it's worse.
The issue is they come from a rather posh background, and they're Brits. I thought at one point of asking for a special dialogue editor, well versed in British public schools. I eventually decided to try to have as little dialogue as possible. And after reading Stephen Fry's The Liar, which takes partly place in a public school, I decided that one of my characters, who acts as the leader had to use as complex a vocabulary as possible. This sets a contrast.
I remember using way more than 800 words when being a teenager, and yes we had phrases, that have long been outdated.
One thing is that JRoss' little dialogue by his sister should be saved and submitted to teenagers in 2020, and will certainly be unintelligible. I didn't understand much myself.
As Graeme said many times, dialogues create the illusion of reality. If you ever manage not to experience TV/ radio for a few months, and then watch a movie, you'll realize how unnatural it sounds. Because dialogues/ speech in the media are conventions, not a reflection of reality.