Good thing this is a manga, setting in fantasy middle age time, where marriage can happen by the time they reach adulthood which is quite young compare to our real life, if not i'm calling the loli police on Shize lmaoooo
Pretty sure 16 is legal marriage age for women with parental consent in japan. So this isn't actually jailbaiting, locally, even if you exclude the setting.
"Seduce her"...? Shize yesssssssssss. Tho the 16 thing is a bit weird but guys she's literally living on her own, started her own business, knows badass battle magic and is considered and adult there so yeah don't see much wrong with it. If it was Earth or she was naive or something then I'd probably not support it.
16 is pretty normal for fantasy stories, even in Western stories. Ariel from the Little Mermaid was also 16, for instance.
Also, this is technically an isekai, though it barely comes up, and she was an adult office worker before. So, she's not really a child...