As a millennial, I will say, the vaaaaaaaaast majority of authors do not write accurate millennials or even *try* to write accurate millennials. Now that I have aged out of Romance Heroine Zone (God forbid a heroine be in her 30s), I am not irritated by this as often, but I do roll my eyes pretty often.
But I think it's pretty basic: write your characters the way they would speak. This can be stylized, sure, but that should be a conscious choice, not an accident.
IME, a lot of editors are overly concerned with dumbing things down for readers, but you do need to know *your* readers and your genres readers. For example, as a romance author, I assume my readers know a fair amount about the life of the average woman their age, about sex, marriage, kids, etc. I don't assume they know much about medicine or film, so if I write a film major, I make sure I reference *very* popular films or write my references in a way where the reader does not need any outside knowledge. (Pop culture references are tricky in general, IMO. They get annoying really fast. Use sparingly).
As for allusions, I have never heard "watch your six" before. I have heard six-o'clock; nine o'clock; etc; directional reference. I think "no such thing as a free lunch" is self-explanatory, but I don't think I've ever heard anyone my age use it... even on the Freakanomics podcast. I do not see a 22 yo and 29 yo discussing it, unless one of them is an economics grad student who grew up with a parent in finance. (I grew up with a dad in finance, and I still would not say it).
Millennials are NOT young people anymore. The youngest millennials are in their late 20s. Young adult and new adult age readers are Gen Z.