Tinkerbell was 9 years old when we added Gizmo, who was 10 weeks old when he came home. Tink was an only dog at the time, but she previously had an older "sister" who had passed away a few months before. She was not fond of Gizmo at all for about 4-5 days, and then she started to warm up to him. She would slowly start to play with him, and would eventually even make the effort to crouch and roll over so he could jump on her, because she was taller than him and he couldn't otherwise. It was adorable lol. Now they're best friends (Gizmo is now 2), and we've just added Finley to the family as well. It takes some adjustment time and every dog is different, but it pretty much always works out in my experience.