Shin Splints (Medial Tibial Stress Syndrome): This is a common condition in adolescents who engage in sports or activities that involve running or repetitive stress on the legs. The pain is typically bilateral and occurs after exertion, making it the most likely diagnosis in this scenario.

Muscle Strains: Overstretching or tearing of the muscle fibers, especially in the calf or thigh, can cause bilateral leg pain after exertion.
Overuse Injuries: Conditions such as stress fractures or tendonitis can result from repetitive stress or overuse, leading to pain in both legs.
Growing Pains: Although more common in younger children, growing pains can occasionally be seen in teenagers and may manifest as bilateral leg pain, often worse at night or after activity.

Compartment Syndrome: A serious condition that occurs when pressure within the muscle compartments increases, potentially leading to muscle and nerve damage. It is crucial to identify and treat promptly.
Osteosarcoma or Other Bone Cancers: Although rare, bone cancers can cause leg pain, especially if the pain is persistent, worsening, or accompanied by other systemic symptoms like weight loss or fever.
Deep Vein Thrombosis (DVT): Uncommon in teenagers but can occur, especially with a history of trauma, immobilization, or familial clotting disorders.

Peripheral Arterial Disease: Extremely rare in teenagers but could be considered in those with significant risk factors such as smoking or a family history of early cardiovascular disease.
Neuropathic Pain: Conditions affecting the nerves, such as peripheral neuropathy, can cause leg pain but are less common in teenagers without underlying conditions like diabetes.
Inflammatory or Autoimmune Conditions: Such as lupus or rheumatoid arthritis, which can cause widespread pain and inflammation, including in the legs, but are less common in this age group and typically present with other systemic symptoms.