According to encyclopedia.com 1 the production of the hormone melatonin (5-methoxy-N-acetyltrypt-amine) varies over the course of a lifetime and over the course of a day.
Newborn babies produce very little of this hormone but after the first few months of life the pineal gland increases its production of melatonin. The highest levels occur in children who are about four to seven years old. Older children and adults produce smaller amounts, as production gradually decreases during puberty. The typical adult male produces about 30 micrograms of melatonin during a typical day.
Also, the environment affects the production of melatonin. Darkness stimulates the pineal gland to produce melatonin, while the presence of light inhibits the release of this hormone. Exposure to (artificial) light during the night will inhibit melatonin production.
Lastly, regarding melatonin supplementation in adults the standard dose used in studies ranges between 1 and 10 mg, although there isn’t currently a definitive “best” dosage. It’s believed supplements with a dosage in the 30-mg range may be harmful 2. While the Canadian Paediatric Society recommends 1 mg of melatonin in infants, 2.5 to 3 mg in older children, and 5 mg in adolescents. 3
3. Cummings C. Melatonin for the management of sleep disorders in children and adolescents. Paediatr Child Health. 2012;17(6):331–3 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3380753/?report=reader