Although these are useful suggestions and answers, I believe @MarkBaker's answer is key. I'm 26 with 3 younger siblings (very much in touch with my childish side) and I believe that there are 4 important sides to this issue.
1. The "dialogue is not realistic", which is nicely summed up by @Mark Baker.
2. Understanding how slang develops is as crucial for imitating or representing it authentically. Different groups of people will develop and use slang that follows patterns that may be unique within each group, and not necessarily conscious (at least to begin with). These patterns may make sense in a certain language (in France some use verlan : https://en.wikipedia.org/wiki/Verlan which follows a mapped out pattern) Translating the french verlan into English, Danish, German etc. would probably not make sense. Sure, you would end up with "weird words" that could keep the definition from the original language, but it wouldn't make sense the way it used to. I'm not saying it has to, I'm simply saying that understanding the language and its rules may be crucial to why that specific slang is used. Looking up slang in the Urban Dictionary as mentioned here can be useful, but you still risk using 'established slang' in a way that isn't authentic to the reader (or isn't relatable to a certain group of readers).
3. Understanding "why" slang develops. Of course language changes over time, but I will focus on a social value of slang. I believe the use of slang is a natural part of young people's process of finding themselves and their place in society (sorry, no sources at this moment). Establishing in-group/out-group relations can be done almost entirely with language use. It may be more relevant to focus on "the average young adult attitude/state of mind" than the exact use of words by any specific group of young people (unless you are actively representing a certain group). Slang can be utilized to a point where even if you recorded a group og people talking, chances are you wouldn't even know what to make of it. Two different groups of young people may be completely unaware of the definitions and uses of slang in the other group. I know most people would understand only about half of the things that my friends and I say - when together. Within my group of friends we have more than 20 words for everyday feelings, events, types of people, actions and so on, and the connection between the words used and the definition it not obvious to out-group individuals, of course. I say 'of course' because we value the fact that we can use these words around the subject of our communication without compromising ourselves.
4 For every word or family of slang words there are people that develop the slang and people that simply adopt it. This is a tricky balance and something that may be worth keeping in mind. Who says what is NEVER without importance. I will give two examples the clarify, the second in particular being based on personal experience:
A: A student in a classroom discusses something "cool" with a another student. The teacher overhears this discussion and joins in using slang he has heard them use before: "Yeah, that dude has so much swag, it's cray-cray!"
The students will probably (consciously or subconsciously) react in two ways going forward: 1: "Wow, our teacher understands us. Respect!" (not usual). 2. Wow, our teacher uses "swag" and "cray-cray". Now those terms are lame!". Don't underestimate the effect of slang becoming mainstream.
B: Two friends use "a new word" or expression within their larger group of friends and one not-as-close friend becomes obsessed with the new word. If this not-so-close friend is considered less "alpha" or less respected, he alone can "ruin" the new expression. This can be either because of over-use (yes, use of slang also has an internal balance) or because that word is now connected to that individual (and the people he represents), which in this case makes it "less cool".
Perhaps you could try a different approach:
Create your own slang for the group of young adults.
Do this based on your (current or in-development) understanding of the "how" and "why" for slang development. Yes, you risk dating your work by basing some of the slang on interests within the group that are "modern", but a lot of slang is timeless. The origin of the slang may be obviously dated, but in my opinion that doesn't necessarily date your work.
Test how readers react to it, especially young adults. If done authentically, I predict they will relate to the group even if they use completely different slang from the readers own group.