I agree: it’s definitely not a first person narrative, but for the most part we are viewing things through Shion. We know what he knows and don’t know what he doesn’t. Which is what makes this description strange to me. Surely from Shion’s perspective it would be more like she looked to be in her twenties. And if it’s more from a narrator’s perspective, it could just be stated that she is in her twenties.
Maybe I’m missing something (in fact, I almost certainly am), but ‘is thought to be’ seems to be a rather odd / convoluted way to describe her.