Bit late to this one, but if you're interested, it paints a timeline if you look at all the recruitment stuff.

First one: No requirement.
Second one: Adults 20-30
Third one: Females 10-25, no health requirements, examined for brain disease
"Human" Characteristics of NIKKE: "Complete human brain, woman in her 20s"

Suggesting that at one point, every man and woman of any 20+ age was a candidate. As the elderly failed tests and/or died to Raptures, they narrowed the tests. At some point, they realized men aren't compatable (pesky chromosomes) and, with a lack of subjects, started recruiting children. Chances are testing on children went horribly wrong on top of being morally bankrupt, so they stopped doing that, and/or humanity is stable enough on the Ark that there's no need.