[targonrotts]

i agree to double check akc rules, as they are always changing. the website is American Kennel Club - akc.org .
as to age, what akc means is up to what age they will register, and after a certain age you need vet documentation that said dog and bitch are the parents. not that they encourage you to breed at that age.
and yes, what is not ok in age for a rott, might still be ok for a yorkie.
if a bitch has never been bred before, after 6 years of age you tend to run into more complications-higher chances of pyometria,infections, non-pregnancy, might not be able to have pups naturally,not a good mom, ect.-regardless of the breed.