My first cavalier was Pippin. I got him at 5 months old and he was already housebroken and past his worst puppiness. He was such a dream that when he was 10 months old, I decided to get another one.
Merry was 2 months old when I got her. So we through all the puppy stages with her. A long haul. Chewing, a lot of accidents inside, not much sleep. I learned that she couldn't go to doggy day care until she was 12 weeks old, so that made it hard, since I work 8 am to 5 pm Monday through Friday. I came to an agreement with my breeder, that between 8-12 weeks, Merry would live with the breeder during the week and I would get her on weekends. Then at 12 weeks old Merry came to live with me fulltime.
Luke came to me at 3.5 years old. Fully housebroken, but has some social problems because he hasn't been around anyone other than family. So that has been harder. He came to me this way and I have to work at getting him past his fear of strangers. We have a lot more work to do on this subject.
Also, I am not sure if I would want to get another adult dog to introduce into the house. I would have the weigh the individual situation very carefully. I know many people on this board rescue cavaliers, but it is a real challenge and responsibility and I am not sure I have the training skills necessary to do this. Luke wasn't a rescue, but he is a retired stud dog and he didn't get the socialization with outside people and things that he needs. I think I can work with Luke to get him past his problem, but I don't think I could handle more serious issues with an abandoned dog. I really don't have the skill set necessary to give the dog what it needs.
Jolly came to live with me at 13 weeks and I purposely chose to wait to bring him home. I could've brought him home at 8 weeks old. However, I did that before with Merry and it is hard housebreaking a puppy. So the breeder started his training and by 13 weeks he had the general hang of things. He could sleep for a longer time at night, so I got my sleep. I take Jolly everywhere, including doggy day care so that he is socialized with other pets and other people.
I think in the future that I would bring a puppy home at 12 weeks of age or older. I don't need them when they are real tiny. I would rather have them a little older, so they are more ready to listen and learn.