My first dog on my own was a shelter mutt. She was awesome. When I got her, they told me she was 9 months old (she looked like a 9 month old shepherd). The thing is, she looked like a 9 month old shepherd until the day she died, so I don't really know how old she was. She had a few oddities about her personality, but nothing major. And she was incredibly intelligent - more so than any dog I've ever known.

Since then, we have adopted two other shelter dogs, one at 4 months old (born in the shelter) and one at about a year old. Both have been great dogs. The 4 month old easily has more 'issues' personality-wise than the older dog, so getting them young does not guarantee anything at all.

In fact, in our 4 years of breeding goats and chickens, it's become very obvious that temperament and personality are as much an inherited trait as they are learned. The last dog we adopted was a 'designer breed' who was bred specifically for temperament and she is like the polar opposite of all of our other animals in terms of personality. Breeding matters, so while you CAN re-train dogs who have had difficult lives/experiences, I am in a firm believer in the fact that some things are inherited and cannot be changed.

That said, the best way to pick a shelter dog is to spend a lot of time with it before you take it home. Both of you. And do this with other dogs as well - so that you can start seeing how different personalities jive with you. In our case, we wanted a dog that is curious, attentive to us, but not clingy. Good energy but not hyper. Does not show aggression towards other dogs and is also not obsessed with other dogs to the exclusion of us. Bright, intelligent eyes, healthy coat and sound movements are also important (health, more than temperament). And I agree that fosters and rescues are excellent resources for your search as well.