Given that comment about muting you'll probably, ironically, not see it, but if you genuinely want to know: the main cue about character ages in anime style are eyes: adults have them much smaller and narrower than teens. You can see this exact effect on the image of the "adult pair" in your game. Compare it to your "young pair" and the difference should be obvious.
Since your game uses AI artwork, specifying "adult" in your queries can help. If you use keywords like "young" or "teen" that's obv. going to be detrimental if anything. And no, you don't need anything silly like mustaches or giant tits. There's plenty of clean shaven and slim adults also in anime, and they're still easily recognizable as such.
The other cues would be relative sizes -- when your "younger pair" is considerably shorter than the "adult pair" with the boy apparently being even smaller than the "mature woman" it makes them look pre-pubescent, like they didn't even finish their growth spurt. Finally, there's fashion choices: pigtails are commonly used in porn as a shortcut/implication the character is very young, and i shouldn't even need to spell out that dressing in "fashion" which is effectively high school uniforms creates/reinforces the same implications. if for some reason you want to insist on Japanese aesthetics consider running different queries for your research, e.g. "casual japanese fashion" will give you
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.hth.
