There is also a comedic 'TNG Season 8' book: Warped. I actually have this one, but haven't read it yet.
> the "how old is Mariner??" question
recently I've learned that Mariner was a Cadet aboard the 1701-D during the "never-aired Season 8 of TNG"
you'll note the book's author is Mike McMahan, the same guy who created and serves as show-runner for LDS. So that material is at least "beta" canon, and I have a hunch at some point on the show, they will finally spell out that yes, she's been in Fleet much longer than the other Cerritos ensigns
I tried tracking down this theory that she was on the Enterprise, and I think it came from Mike McMahon's original TNG Season 8 tweets. He expands on them in a book and creates a new lower-deck character named Beckett Mariner.
He also says in the book, "There's no chance I'll ever get to write for a Star Trek show (which is probably for the best)." (page 270) He was wrong about this.
Beckett Mariner would clearly be lifted from that book as inspiration for the actual, canonical Lower Decks show, but any there's no guarantee that the show Mariner IS the same as the Mariner in McMahan's original fan content.
That being said, associating Mariner with a previously-seen but never-explained TNG character is super cool headcanon!
He was actually licensed by CBS to publish a book version: https://www.amazon.com/Warped-Engaging-Never-Aired-Season-Generation/dp/1476779058/