Answer
The co-regent of Shapur I until 242 was his father, Ardashir I, the founder of the Sassanid Empire. Ardashir I had designated Shapur as his successor and co-ruler in 224, ensuring a smooth transition of power and consolidating the Sassanid dynasty's rule over Persia. This co-regency allowed Shapur to gain valuable experience in governance and military leadership, preparing him to independently assume the throne after Ardashir I's death.