Musculoskeletal strain or overuse injury: Given the age and the duration of the pain, a musculoskeletal issue such as a strain or overuse injury from physical activity is a common cause of thigh pain. This could involve muscles like the quadriceps, hamstrings, or adductors.

Tendinitis: Inflammation of the tendons around the thigh, such as the quadriceps or hamstring tendons, could cause pain, especially if the individual has recently increased their physical activity level.
Stress fracture: A small crack in one of the thigh bones (femur) could cause pain, especially if the individual has been engaging in repetitive stress activities like running.
Osteitis pubis: An inflammation of the pubic bone and its surrounding structures, often seen in athletes, could radiate pain to the thigh area.

Deep vein thrombosis (DVT): Although less common in young individuals without risk factors, DVT is a potentially life-threatening condition that could cause thigh pain and swelling. It's crucial to consider, especially if there are risk factors like recent travel, immobilization, or family history.
Osteosarcoma or other bone tumors: While rare, bone cancers can cause persistent pain in the thigh and must be considered to ensure timely diagnosis and treatment.
Infection (osteomyelitis or septic arthritis): Infections of the bone or joint can cause severe pain and are medical emergencies, especially if accompanied by fever, redness, or swelling.

Peripheral artery disease: Although more common in older adults, peripheral artery disease could cause thigh pain, especially during exercise, due to poor blood flow.
Nerve entrapment or compression: Conditions like meralgia paresthetica (lateral femoral cutaneous nerve entrapment) or compression of the femoral nerve could cause thigh pain, numbness, or tingling.
Bone cysts or benign tumors: Such as unicameral bone cysts or giant cell tumors, which are rare but could cause thigh pain due to their location and potential for fracture.