So nobody's gonna mention how this actually implies that the goldfish girl is probably gonna die young?
That doesn't necessarily follow, depends on what the internal rules of the world are. The tradeoff for fast maturation might simply be always being loli-looking, not having a short life. Or it might be frailty or something else.
Clearly the author probably didn't work all that out, but fast maturation doesn't necessarily mean accelerated life cycle (although it could if the author wanted it to).