(N/A) Spermatogenesis starts at the age of puberty due to a significant increase in the secretion of gonadotropin-releasing hormone $(GnRH)$. This is a hypothalamic hormone.
The increased levels of $GnRH$ act on the anterior pituitary gland and stimulate the secretion of two gonadotropins: luteinizing hormone $(LH)$ and follicle-stimulating hormone $(FSH)$.
$LH$ acts on the Leydig cells and stimulates the synthesis and secretion of androgens. Androgens,in turn,stimulate the process of spermatogenesis.
$FSH$ acts on the Sertoli cells and stimulates the secretion of certain factors that help in the process of spermiogenesis.