I do get it, but I also think it's important to remember that Stella is an alien. The given age could be wildly different compared to humans. At least that's what I choose to believe to make it less weird.
Unfortunately, even if you were to completely ignore Stella's listed age, she's still noticeably dumber and less mature than most of the other cast members, and far more reliant on Gimlet's guidance compared to Oriko & Sebastian, or even Midori & Sumire. You also get moments like in chapter 62, where Midori expresses remorse at having pointed a gun at Stella specifically on the basis of Stella being "a child". And in chapter 76, she directly compares her age with Giraffe's, which wouldn't make much sense if her years were different than Giraffe's years.
Speaking of Giraffe, he gets into positions that are
way more compromising than anything that happens to Stella (case in point - the chapter that came literally
right before this one), is even younger than Stella, and can't hide behind the "alien years" excuse at all, so if this is the kind of stuff you're worried about Stella is honestly the
least of your concerns. And even Oriko herself is only 16.