It depends on what dog breed you have. I own a Golden Retriever and she is 10 months. We havent been jogging with her once as personally I think its to young. I would recommend you start running with them at 18 months and onwards. If you find this to late then start running with them at 1 year but no younger. Hope I helped.
The reason for waiting to jog with the dog (as opposed to just letting the dog run on its own in the yard or park on grass) is to wait for the growth plates to close. Basically, for the joints to be physically mature. The younger the dog, the more the repetitive impact of running can damage the still immature joints.
Smaller breeds mature faster than large breeds and giant breeds mature the most slowly. So it would be reasonable to start running with a JRT at about 12-14 months but you wouldn't want to run with a large breed until 18-20 months.
If you are going to do long distance running and want to start building the dog up for that, it might be prudent to have x-rays done at about 16-18 months of age to see if the dog is fully grown (and if there are any obvious structural faults). And when you do start to run, like for a human athlete you will want to build up the dog's endurance slowly and be on alert for heat related illness (which of course dogs are more susceptible to than the typical person due to cooling mechanisms).