re: Mariner's age VS her appearance, you have to be careful about using appearances as your yardstick because this is an animated series, and it seems to me like the show is really using visual cues not to show age, but to distinguish the command staff from everyone else. An actual age cue would be Captain Freeman's streak of gray hair, but body types and facial structure is all over the place, with most of the crew looking not too dissimilar from Boimler or Tendi (and do we know exactly how Orions age? Season 2 hinted Tendi has more past in her past than she lets on, after all). Rutherford is unusually stocky compared to most of the crew, being beaten out only by Shaxs for biggest body type on the ship (...not counting the beluga officers, of course). So clearly Mariner is within the expected age range for a Starfleet Ensign. Remember how Picard's life could have gone if he didn't get stabbed in the back in his twenties? Lieutenant Junior Grade at the age of 60. He made Harry Kim look like he was going places.
And if we don't account for artistic license for this being animated, she could still look plausibly like she was around the same age as Boimler when she isn't. I'm in my early 30's and strangers still mistake me for a senior in high school all the time. I'm not kidding. Somehow my face and body type takes more than ten years off me in the eyes of the general public, unless they stop and think about how many highschoolers they've met with a full beard. That's why bars, casinos and liquor stores need ID, because you can't always tell from looking at a person how old they are. Its also the basis for the common advice not to ask a woman their age. I can completely see Mariner being in her 30's, especially given the implications about her mother's age, and yet looking at a glance like she belongs in the same cohort as Rutherford and Boimler.
The scars also support her having a fairly long career and having seen combat on multiple occasions, even if she's prone to making up stories to impress people. Remember the second episode? She said that she knew the Klingon General because she did off the books Grey Ops work during the Dominion War. Grey Ops isn't what it sounds like, Grey Ops means psychological warfare or propaganda work. No doubt the joke is that Boimler, much like the majority of the audience, wouldn't realize that's what that meant and would assume she was doing
covert ops with the General-- who notably didn't correct her, so her claim about doing such work has some evidence behind it. Probably she was stuck on a Klingon Bird of Prey cloaked somewhere near Cardassian space covertly broadcasting Federation propaganda to try and start an insurrection against the Dominion. Would also explain her posting on DS9 at the time-- and why she was never noticed by the DS9 regulars (it
was supposed to be off the books, after all).
Even her having more energy than your typical 30 something is easily explained by Federation medical science. Remember, they considered Picard to be in the prime of life
in his 60's, and McCoy was still alive to give the Enterprise D a visit in the first episode, remarking to Data that there should be nothing surprising about a man not dying. It shouldn't be that surprising if the same anti-aging technology allows even younger people to keep the energy of a twenty year old for longer as well.
As for her behavior, yeah, I concur with those who say age and maturity aren't always directly correlated. Its clear that something happened during her time as a Lieutenant that left her jaded with Starfleet (probably all that Grey Ops work-- nothing makes you lose more faith in the system than having to tell official lies on its behalf, like that Starfleet is winning a war it is most certainly losing
). So that alone covers a lot of it. She has no more f$#&s to give and that's taken a load off her shoulders. And as a loner, Mariner would have an easy time explaining why she doesn't always know what is considered hip among fresh to young ensigns who haven't yet realized that she has almost ten years of experience on them and used to hold the rank of Lieutenant. The rest probably have the same attitude as Jennifer Sh'reyan: "I don't think about you at all."