Hello, - The baby bones are a miniature replica of the adult bones. However, they are comprised of cartilage scaffold (soft mould)which later gets stronger by deposition of calcium and bone forming cells. - There are growing centers at each ends of the bone called growth plates. These plates keep depositing bone cells and calcium and other minerals. - They grow longitudinally and horizontally as well. The Bones get stronger with daily activities and stresses borne by them. -This transformation from cartilage to mature bone is monitored by various parameters like: -Diet -exposure to sunlight for Vitamin D -hormone- growth hormone, thyroid and parathyroid hormones etc -genetic make up -calcium and phosphorous levels in the body. -presence of any systemic disease. Hope that solved your doubt.