Where it all began
The Numan race made it's entry into the Phantasy Star world in Phantasy Star 2. The hero of the tale, Rolf, has a best friend; a biological construct whose origins are not known.
Nei differentiates herself from the other characters early by advancing through levels faster then any of the Palman characters. Nei's origins, however, only reveal themselves upon entering the Biosystems lab, one of the creations of the ruling A.I. Mother Brain, protector of all Motavia. Within that lab the party encountered NeiFirst, the prototype created by Mother Brain before creating Nei herself.
NeiFirst had been infected by the same dark force (hint, hint) that had infected Mother Brain itself and thus the party battled her; resulting in Nei and Neifirst's death. Apparently...
Fast forward to the post-PSIV present day, and we have the "ultimate" Numan, Rika. The result of a millenium of research, she is released into the real world by her creator Seed, to join up with Alys and co.. Again, she advances quickly and has strength in techniques like her predecessor, and also is at age 1, as Nei is. As far as lifespan goes, the life of a Numan could be short or long, being biological constructs, but the only three in existence grew to full maturity within roughly the space of one year.
Rika differed from her predecessors, however, by firstly surviving the conflict, and secondly, falling in love with our main protagonist Chaz. They went on to form a relationship after the events in the Edge, the first Numan/Palman relationship ever recorded. Where they went with this relationship, and if it bore any children, is known only in the fevered minds of Sonic Team or SEGA next time they get Phantasy Star out for a proper sequel to PSIV. Although it's possible that they did have children, and whether this lead to the Numan race in PSO, is unknown, although I believe that it's a possibility.
The NeiChildren differ from Rika and any offspring she and Chaz may produce simply because NeiFirst is a prototype of a prototype, if Seed's program for creating Rika is descended from Mother Brain's.

In the case of the NeiChildren, due to the randomness of technology and the unfinished nature of Nei and NeiFirst, their lifespans are indefintite. Some may live up to hundreds of years, while others may fail of strange degenerations early in life. Regardless of how long they live, their outward appearance degenerates much slower than Palmans, due to their biomonster cells; thus, for example, Zedd could be anywhere between 1 and 50 years old, currently. Their longevity has a downside, however, as the small amount of male NeiChildren born means that their numbers are still low, even after a thousand years...

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