It says Heisei 17, May 3. That’s her birthday, not her age.
Did you really think an official document would ask for someone’s age instead? Like, the marriage form is only valid while she’s 17 and stops being valid as soon as she turns 18 or something?
I think in today's landscape, in particular with online communities of degenerates. You're likely to hear more people wishing that they could have a cute girl who has some yandere levels of interest in you who prioritizes getting to be with you over the worries of societal acceptance or even the lives of others.
Though yeah, in reality having someone with that sort of potential sociopathy or psychopathy and manipulative capabilities would be more worrysome than not. Good thing this is a work of fiction for our entertainment.