Actually, if she's 18, she has to have been something else before she was Yamataian; the oldest 'natural born' Yamatain is three, as the Yamatain race was created in the year 27, and it is currently the year 30.
Yamataians were created to replace Nekos as the 'standard' race of the empire. Nekos are engineered more for war than for civillian life, but because of constant conflict and the fact that they can be manufactured quickly, Nekos came to outnumber every other race in the empire.
As such, it was decreed that the Nekos be 'reintegrated,' and the baseline human form was revisited in the Yamataian race. Many civillian Nekos (and probably almost all Geshrin) were converted to Yamataian bodies.
On top of this, it was decreed in year 29 that all members of the Star Army of Yamatai must be either Neko or Yamataian, so most service members who were not of these races either converted to Yamataian or were discharged. Renko had to have either been converted earlier in life or upon enlistment.
Also, there are no records of 'old earth' anywhere in the universe of Star Army of Yamatai- or at least nowhere that any player character could get access to them. The civilization and culture closest to the 'old world' of humanity is probably Nepleslian, which might be what Isuzu has been picking up on.
Another thing to consider: Nekos and Yamataians have 'digital brains' that can telepathically access computers (or other Neko/Yamataian brains) and store information in organized, interactive, non-degrading file systems. Geshrin and Nepleslians (the two 'baseline' human races in the setting) do not.
One notable difference between Ren and Isuzu's arrivals was that Ren was trying to find her way on her own while Isuzu had taken the precaution of having a complete schematic of the ship downloaded directly into her brain, which she was using to get around. This difference of approach is probably pretty typical when comparing Yamataians converted from Nekos with Yamataians converted from something else ^_^;.
When characterizing cultural norms among humans, it might be instructional to think of Geshrin as 'Japanese' and Nepleslian as 'American' or 'European.' As i understand the setting, Nepleslia came first, and then the founders of the empire of Yamatai split off from Nepleslia at some point. Most of these founders were of 'Japanese' and 'Nordic' cultures. These splinter Nepleslians eventually became 'Geshrin' through genetic engineering.
Later, Yamatai came back to conquer the Nepleslians, and Yamatai ruled Nepleslia for a time. Nepleslia has recently regained their independence, however, and their current relationship with Yamatai is barely civil, but there are probably enough Nepleslians still within the Empire of Yamatai that Renko could have been either Nepleslian or Geshrin, or a mix of the two.