No dog is too young to develop a cyst, abscess or tumor. Young animals, just like children, occasionally develop cancers.
If your dog has a lump that is growing, you should try to identify it. Under a short-acting anesthetic, the mass can be radiographed to see if it is attached to the bone. If it is not attached to the bone, the mass can be tapped to see what is in it. The material can be stained on a slide, or a piece can be sent to a laboratory for a pathological diagnosis.
If the mass turns out to be scar tissue or a cyst, it is safe to take a wait-and-see attitude. An abscess would need to be drained and perhaps treated with antibiotics. If the lump turns out to be a tumor, the pathologist will report whether it is benign or malignant. This information will help you and your veterinarian decide the proper course of treatment.
If the tumor is benign, the best course may be to remove the mass while it is still small and the urgency is not great. A malignant tumor, on the other hand, would require radical and immediate surgical removal, perhaps accompanied by radiation or chemotherapy. These decisions can be made only after proper diagnosis, however.
If the diagnostic work is inconclusive, your veterinarian can arrange for a second opinion. He or she can consult by phone or send the X-rays and lab work to an orthopedic specialist or an internist. Or you can be referred to a specialist, probably at the veterinary teaching hospital at the nearest veterinary school.