Regardless of whether this idea is "new" or not doesn't make it any more believable or less stupid. The thing that makes this work so irritating and others less so is that if you're going to break with accepted reality in fiction there needs to be an explanation, reason, or internal logic to it. E.G. if an ecchi series has a bunch of "high school girls" who are all drawn with tiny waists, DDD tits, and skimpy costumes, it's easy to understand: the author is working in a particular genre and the way he draws the characters is in service of that. This just reeks of a lazy fetish indulgence - the author just has a thing with the idea of a "hot" elementary school girl. It's not about why this girl is unusual, it's about the other woman's inappropriate attraction. Perspective and reasoning matter if the audience is going to accept your bullshit.
Also, just because you can come up with a (barely) plausible explanation, doesn't mean that the situation as shown is plausible. It's the author's responsibility to explain something this outlandish if it's not intended to be straight fetish.
And to the person questioning whether or not a year is a big deal: I mean, first off there's the obvious answer that one increment in the little number next to the "age" line can mean the difference between jail and not, between being allowed in or allowed to buy or allowed to do and not. But on a more scientific level, a year's time growing up means some big physical changes. The difference between an 11 year old and a 13 year old can't be dismissed. (And I mean that just in terms of their physical appearance, not acceptability for attraction. The obsession with 14 as the "ideal age" is a whole other discussion. But at least being post-pubescent and at the age of consent is... something?)