I think it would be fairly rare for a 20-something female “influencer” to say she was 31+ unless her age was part of her schtick. In this case, I wound have expected her to lie the other direction — she could pas for younger than 25, so “Heisei ??” seems like it would have been more likely.
The shift from 20s to 30s for females in Japan is generally not a welcome one — pressure to marry, have kids, “settle down”, etc. are very real. It is also perceived that women in their 30s are not as “cute” (something often sought after, especially by public entertainment figures).
Double edged sword. The shift from 20s to 30s also reduces attention from perverts and weirdos, just as it does from gentlemen suitors. And if someone joined a site back when they're a minor they often had to lie.
Or if someone is just privacy conscious about details that may be used to steal their identity, they may have just randomly chose January 1, 1970 or whatever was the top choice on the little menu.
The shift from 20s to 30s for females in Japan is generally not a welcome one — pressure to marry, have kids, “settle down”, etc. are very real. It is also perceived that women in their 30s are not as “cute” (something often sought after, especially by public entertainment figures).