My information (no proof) is that Mourning Mayo was born 1766 which would have made her 17 at Sarah's birth. Birthdates for her children are: Sarah-1783, David-1794, William- 1799, Reuben T.-1801, Mary M.-1803, John-1806, Samuel-?.Mourning & John, Sarah & Gilliam are buried at Sandy Creek Baptist Church, Butts Co., GA. Perhaps someone could check out their tombstones for dates. Regarding the several years between Mourning's first children, there could be several answers, such as miscarriages, illness, etc.
I don't know about the many Mayo, Richardson, Floyd marriages. It seems confusing at the moment, rather like an interconnecting maze. Hopefully, someone can decipher it all and let us know.