In Unani medicine, the stages of life are divided into phases based on the balance between anabolic and catabolic processes.
Sin-i-Numu is the age of growth where anabolism exceeds catabolism — the body builds more than it breaks down.
Sin-i-Shabab is the age of youth or maturity where anabolism and catabolism are balanced — the rate of formation equals the rate of destruction.
Sin-i-Kuhulat is the age of decline where catabolism starts to dominate, slowly exceeding anabolism.
Sin-i-Shaikhukhat is old age where catabolism overtakes anabolism, leading to physical degeneration.
Therefore, the stage where formation equals destruction is Sin-i-Shabab.