In the wild, I'm not sure, probably 3 or 4 years. In captivity, I would say the average for a well cared for dragon would be 5 to 7 years, though I know of animals who have lived much longer. And, with some of the advancements in husbandry over the last 5-10 years, it will be curious to see how that affects the average lifespan of captive pogona.
For breeding, over breeding a female can certainly shorten het lifespan. Males, it doesn't matter as much. Men are pretty much men, no matter the species.
As for groups, beardies are certainly social critters. But they seem to do as well socializing with people, maybe better. If you have dragons that get along, then its good. But each situation is individual, and beardies can fight, so you need to watch them.
Actually, all of our beardies are adopted out. I've never had one die on me, but I've never kept one that long either. I'm really not a bearded dragon guy. I just try to help out because they have become the reptile equivalents of overpopulation of dogs and cats in the pound.
The fighting is most common between two males, but it can happen between any dragons
