TBF "looks" doesn't necessarily correlate with actual age - never mind now how whatever bizarreness is going on with Amiya's Infection (that unique "Chimera" race append doesn't exactly suggest physical normalcy) might further screw things.
People who know her well keep talking about her as quite young tho which I would consider rather more reliable indicator.
Either way, everyone says she's a kid, from the characters close to her to completely unrelated characters. Also another related thing is Amiya's height.
Other Cautus's heights in order from tallest to shortest:
Ayerscarpe - 173 cm
Leonhardt - 165 cm
Savage - 160 cm
Rope - 155 cm
Kroos - 154 cm
Amiya - 142 cm
Conversely, here's the heights of child operators:
Vermeil - 153 cm
Eyjafyalla - 145 cm
Popukar - 144 cm
Amiya - 142 cm
Suzuran - 137 cm
Even height-wise, she's clearly leaning on being a kid. You could say she's just short, but coupled with everyone saying she's a kid, maybe she's actually just a kid hmmmm...