All three of my dogs (two current and one past) came from the local Animal Shelter. The youngest age of any of the available dogs is about 10 months (from what I've seen during my adoption trips). One was a border collie mix, the two current are a pit mix and a german shepherd mix. My pit mix is the sweetest thing ever and everyone who meets her falls in love with her, usually gushing baby talk to her. What a dollbaby.
The good thing is that you can still get them young enough for them to blend into the household.
The bad thing is that you've pretty much totally missed the window to really socialize them with other dogs and people (you've gotta get them and train them when they're 3-6 months old tops). All three dogs had/have issues with bonding with other dogs and/or people. They're getting training to correct it, but it is an issue.
I love the fact that I've rescued my puppies from a horrible fate and I know they are so grateful to be a part of my family. I wouldn't trade my babies for anything. But dogs that have been abandoned and unsocialized as pups can have problems. Of course, some breeds are naturally friendly so this caveat doesn't apply.