In the process of what does age-related variability of the organism occur?

Age-related variability of the organism occurs during the development of the organism. All changes are provided by the action of hormones, which are produced with a certain intensity at certain periods of life. For example: Samotropin is a growth hormone that is produced by the thyroid gland up to age 20. Sex hormones begin to be actively produced at about 12-14 years old. The development process ends at about 23 years old. Of course, all hormones are produced further, but not in such volumes and with less intensity.