This is a derogatory slur made towards those who are born rather than created through AGEd processes. In some cases one might not be able to tell, but there are giveaways such as brown hair often is a symbol that someone was born.
Solarians can still procreate normally, and two AGEd parents who conceive a child will produce a ‘Birther’ child because the child was not created. In many cases a ‘Birther’ could have just as good of genetic potential, but the slur generally implies backwater planets and substandard genetics.
Many people who are not AGEd later go through Gene Therapy to bring out their best potential.