[LunarBerry]

My girl was 8 weeks when I got her, also a bit young but I had older dogs to help in her socializing and I took extra steps for her to adapt. Granted she did beautifully when she was young, going on the pad and all, doing good on everything else. But once she got older it got a bit trickier/harder. I'd say she's started to go outside well enough at around a year old, with taking her out regularly. In fact, she gets irate or throws a tantrum when she doesn't get taken outside with the older dog(s).
It really depends on the pup of course, I've just lucked out very well. The best thing is to try and make a consistent routine.