HGH is released in pulses. The size and duration of the pulses vary with time of day, age, and sex. This is one reason why random HGH measurements are rarely useful. A higher level may be normal if the blood was drawn during a pulse while a lower level may be normal if the blood was drawn around the end of a pulse. Hence, measuring the levels of IGF-1 is a reliable method of assessing HGH levels. Since GH levels vary throughout the day, IGF-1 levels are a reflection of average GH production.HGH declines with age among all animal species. In humans, after the age of 20-30 HGH levels decline 14-15% every 10 years; by the age of 60 HGH is usually less than half what it was at 25. Below is a graph  to illustrate the levels of HGH secreted by the body  as we age.https://medlineplus.gov/ency/article/003706.htm