I am no vet, and I have no qualifications to reply to this other than my personal experience (4 dogs' worth)
but I think it depends A LOT on the type of surface you run the dog on.
I would NOT run a dog on concrete sidewalks or even asphalt roads until they are well over a year and even then I wouldn't do it.
But I would run them in the woods on dirt trails STARTING at about 7 months, although not every day.. probably 3-4 days a week at that age.
And I would WORK UP to that mileage over a couple of months.. 5 miles has worked for my dogs none of which has had any joint problems (the first ran until she was 10'ish, died at 12 of cancer, never joint problems, dog two ran until 2 weeks before she died at 10 of complications from myasthenia gravis, no joint problems, third dog is 5 yrs old and OFA good, elbows normal, fourth dog is 18 months old and a good runner).
Finally, I run them in an undeveloped section of land (no roads anywhere) off leash so they can gauge their own pace. I carry water and if they fall behind, I'll slow down to their pace.
Like human athletes, you have to get in condition first before you log those miles.
One should also be VERY concerned about heat exhaustion when the weather warms up, especially with those black dogs..
Bente