I would not advise a 6 year old bitch be bred from if she has not had a litter before - she is too old to be a first-time Mum in my opinion. That's not to say that she could not have a litter but I personally would not risk a bitch this age. If she has had a litter before, then it might be different but not if this was a first litter - the risks are too high (older bitches are more likely to have a difficult birth & need a caesarian, particularly if this is a first litter)
Jane