I've had 4 B's so far. Rocky was a year old when we got him, Adrianne was 6 months, Maggie was almost 4 (when we got her back) and mia we got at 8 weeks.
Rocky had HORRIBLE anger problems, and food aggression issues. he still kinda does to this day. (He will soon be 4) he's VERY skittish around new people. and doesn't like to be touched and has sent my boyfriend to the hospital after he bit him. He's aggressive towards our other two B's also.
Adrianne could NOT be potty trained. you could walk her for 15 minutes, she wouldnt go, and then she would go as soon as you took her leash off inside. Nothing seemed to break her of her habit. Her and Rocky came from the same place. (A-bug passed away a few years ago).
Maggie was rescued back from my boyfriend's ex. Most of you know about that situation from the thread i made when we first got her back. She still has trouble with pottying in the house. No where near as frequently, maybe once every week or two, so she's progressing but it has taken a year. I take her out at least every 2-3 hours to make sure she goes.
Mia, the only one we've raised from a puppy, is probably the best of the 4. She WILL NOT potty in the house. She will cry and whine and chase her tail to let you know she has to go. She is much more loveable than mag or rocky, maybe because the other two were neglected so much.
I guess what that shows, in my case at least, is that if an older dog is neglected or abused in a previous situation it may be harder to get them integrated into your household and trained. You do not know how the previous owner may have done things. from a pup you can make sure to enforce training issues and they do pick up quickly, at least Mia did. Mia also bonded to Dan and I within a few days. Even now she follows us around and has to be right next to us. The other two are fine being alone or in a different room.