I've taken Jackson since he was about 5 months old (only because I did not discover it until then). We LOVE our dog park... we have two that we frequent. I definitely think the dog park has helped him be such a social little guy. He loves going there, some days he likes to just chill, others he enjoys running like a crazy man, others he is more mellow, it just depends on the day, the weather and the dogs there.
I would preferably wait until he gets his rabies shot, just to be on the safe side. I allowed Jackson to go outside as early as 9 weeks but dog parks and pet stores sometimes carry diseases I'd be more cautious of than just our backyard, etc.
Dog parks around here are wonderful, most owners are great, pick up after their dogs, if their dog is being aggressive they will immediately take them out. Sure there's been a few scuffles here and there but I love going and don't know what I'd do without it!
Here is one day at our dog park....