I am asking this question,hoping to get anwsers from knowledgable people.
Even though legally through the kennel club they will register a litter from a female that was mated at 12 months old.
My question is in mastiff breeds that are slow, maybe late developers to a lott of other breeds i.e Terriers.what is the best and morally right age to breed a mastiff.
Ive always told people the best time is after 18 months old the nearer to age two the better!.But it is obvious in one newish  mastiff breed on the import registerworking group there is many females getting mated up at 12-17 months old ,by stud dog owners that are established in the breed.Can we have some comments on this why it is better to wait, because people think if the pups can be registered its more than ok to mate their french mastiff up at 12 months old.
  

All breeds vary as to their age at maturity.  I think the best guide has to be the breed club as that is the largest consensus within a breed that you are likely to find.  I don't know about the French Mastiff but the English Mastiff breed club spell out what is expected in their code of ethics clear enough I think :)

The ddb club recomend a bitch should be at least 20 months old when mated and not past the age of six.
I would go with that.
One of my bitches is nineteen months old at the moment and she as only just come in to herself these last few months.
They are very late to develope. Don't be mislead by there size.

Also forgot to say that they are still have a very puppy attitude, it is not just body that they are late to develope, it is mind as well and a young bitch such as twelve months old my not feel she could cope with pups so young.

I have heard it's two years old really especially in bigs dogs they grow so fast then when they hit one they fill out, it's not really in their best interested to breed them at 12 months as they are still maturing.  The kennel club probably just use that as a guideline for other breeds who mature fast and have less complications, most breeders who specialise in the big breeds now what is the best age to breed their dogs, whereas the kennel club is just an organisation, in saying that dont you get their hips scored at one and this takes a while for the results are the breeders you know doing health checks on their dogs before they breed them at 12months