
As others have said, a lot depends on the individual dog - and the circumstances of their initial raising by the breeder.
I have a 6yr old dobe bitch who came to me almost a year ago (end of this month), with fear aggression to dogs caused by lack of socialisation. To the best of my knowledge, she didn't socialise Soli at all - there may have been one or two puppy classes, but her old owner said that Soli never came out from under the chair so she didn't go back. After that she was basically isolated, kept away from kids "just in case" and dogs (except for her litter brother who her old owner's ex had (bought at the same time)).
In the year I've had her she's come on leaps and bounds - sometimes she can meet other dogs, she is always edgy but improving. And she';s much, much better with people now - she used to be iffy with kids and adults, but today she's been badgering people in PAH for fuss. :D
But underneath all that progress, she is still an unconfident dog - and I suspect she always will be. I don't have many details of her first 8 weeks at the breeders, but from what I understand she was raised in a barn - so possibly not much socialisation. Of course that's speculation on my part; at some point I'll get around to calling her breeder (got the number recently) then I can find out a lot more - such as those first weeks, her parents' temperaments and so on.
My brother, on the other hand, rehomed a 2/3yr old crossbreed bitch (most likely dobe X rottie X staff), who'd been in kennels from about 6 months old and was stray before then. She had seriosu fear aggression on-lead, and terrible social skills off it, but still made good progress when I last worked with her (she was 9 then and had a good 6/7 years of aggressive behaviour behind her). She was fairly confident, which helped. But now she's been on her own again for a year (was living with two of mine for a few years before I moved), and when I saw her in October with one of mine (her favourite) her social skills were atrocious - she's an outright bully now, and a very, very OTT one at that. Opi wasn't ahppy at all, so persistence and consistence are big factors in helping these dogs, putting aside their individual histories.