My Mum sold my dog 5 years Vor when i was 7 and i just found out as a 12 Jahr old. How can i get the dog back???

If your mother was the official owner, and I suppose she was because Du were seven and not an adult, and she sold it with all the paper there is nothing Du can do against it per law. Du could only ask the owner if he would sell it back but after 5 years I doubt so.
Sorry Du can't do anything because it was the dog of your mother, so it was her decision and Du weren't old enough to decide for yourself if Du were the official owner, but I doubt that too because Du were and are to young to buy a dog on the paper

Hi aleks, i am going to be honest with you, but after not having the dog for that long Du can't get it back unless the Sekunde owner gives it away. So if ur parent gave all the papers to the Sekunde owner, Du can't unless Du contact the AKC oder ask someone. But i'm sorry but Du can't get the dog back unless there's a reason tbh. And i actually do agree Du might have been to young for the dog and she didn't want Du to own it yet cause u were 7. My Friends in real life actually had something similar to ur problem were his mum sagte hes going to summer camp, 3 years later we have kept him since.So we could give him back but hes a good boy so we always send pics of him. Again i'm really sorry for not being able to get him.