Okay, Yamataians have only been around since YE 27... so, if you were born a Yamatain, you'd be around 2 years old... not 7! ^_^;
There are really two ways to become a Yamataian:
- Be a Nekovalkyrja whom changed her body type to be able to integrate as a civilian in Yamataian society (as in, the inhabitants of Yamatai or for any colonization project requiring settlers).
- Be a Geshrin whom upgraded his body type to Yamataian (given that the Yamataian body is really the 2nd generation 'Geshrin' body). This would be considered normal, as most of Yamatai's population are seen as bio-synthetic constructs. Swapping boides is rather easy, in fact, it's been shown that obtaining a new yamataian body is about as mundane a process as getting yourself a new pair of shorts.
The Yamataian is made for civilian life with nekovalkyrja benefits. It's also much better suited for operating on the Ketsurui Fleet Yard Nekovalkyrja warships (commonly used by the Star Army of Yamatai) as many of them have zero-gravity passages with sentient computer systems - the Yamataian's capability for anti-gravity, telepathy and it's computer-like mind interface helps it a good deal there.
Generally, age is a factor based on the actual age of the personality inside the body and not the body itself. Nekovalkyrjas don't really have a childhood and start off with the mentality of a late teenager/young adult; so, them being 'young' (seven years old or less) isn't really that big an issue with the knowledge and mindsets they are provided with. Nekos can be considered as adults around their 4-5 years old and are defenitely mature by 7-8 years old, which is about the time when they can discontinue their military service and get on with their lives however they choose.
Geshrins, aside from their 3 month pregancies, grow like humans. Most of them would probably be in the 18 to 28-ish age range depending on how early they were born... or if they were soul transfered from the original Nepleslian settlers, perhaps older. All could have switched from their original bodies to the Yamataian body.
If you look around, you might see some players still being Geshrins on the account of being sentimentally attached to their bodies, though this is really the exception more than the norm. Yamatai's lack of belief in the afterlife and disdain for religion discourages attachment - attachment to a body might have the emotional import of it being like 'your favorite pencil' rather than a case of: 'This is me and I wouldn't be the same without this body!' Yamatai's amazing amounts of resources, bio-engineering science and capability to duplicate a person's mind makes it so that, in a way, life
is cheap.