we have got a 6 month old dog.and a 3 year old bitch.at what age will my dog be able to mate with my bitch.

NOW!!!

now BUT normally best to wait untill the dogs 18mths really

saying that for a first litter from a bitch I wouldn't have it once they are over 4 years old!

Rox

Hi Guest, for anyone here to give the most appropriate advice, it would be better for you to join Champdogs, (it free) and provide a more detailed picture. Useful info would be, what breed are your two dogs? are they KC registered, do you show them? have they been successful? have they had the breed relevant health tests done and passed or scored well? You see lots of things will influence the advice.
The short answer to your question however is this. At six months old your dog puppy is physically capable of mating your adult bitch, but is it a good idea? Thats another question that we can help you answer if you register and chat more.
Dawn R.

I was kind of hoping the guest was asking so that he knows whaen he will have to seperate them when the bitch is in season not that he/she wanted to use him deliberately on the bitch...
Archer

The thing is Archer, if that's not the question, and this poster wants to breed at the earliest opportunity, your NOW!! reply could be giving him the go ahead.
I would seriously like this guest to join up and discuss the ins and outs of breeding dogs, and I'm interested in finding out more.
Dawn R.

I appreciate that but maybe sometimes we should not immediately think the worst of an innocent question as we are always getting moaned at for putting people down and not giving helpful advice.Next time I will take more time to explain my answer more...was in a bit of a rush at the time!! 
Archer