XL Bullies typically reach their full height between 12 and 18 months of age, but they continue to fill out and gain muscle mass until they are 2 to 3 years old. Their growth varies, influenced by factors like genetics, diet, and exercise. Genetics play a significant role, and looking at the parents can provide insight into the dog's potential size. While most of their height is achieved within the first 18 months, the broader, muscular build that defines the breed continues to develop well into their second or third year. Proper nutrition and balanced exercise are crucial for healthy growth. Consulting with a vet if growth seems unusually slow or fast is essential to address potential health concerns. Overall, patience and attention to their development will help ensure a healthy and well-formed adult XL Bully.