Max Planck formulated the quantum theory of light, which marked the beginning of quantum mechanics, in the year 1900. At that time, Planck was 42 years old. He published his groundbreaking work on blackbody radiation, which introduced the concept of energy quantization and the Planck constant, on December 14, 1900. This work laid the foundation for the subsequent development of quantum theory and had a profound impact on the understanding of the behavior of light and matter at the atomic and subatomic scales.