Final answer:
Disequilibrium in a family during adolescence is often caused by the beginning of puberty and sexual maturity, along with significant psychosocial changes.
Step-by-step explanation:
The key change which may cause disequilibrium in a family as a child enters adolescence is the beginning of puberty and sexual maturity. Adolescence is a period of significant growth and development across various dimensions including physical, cognitive, and psychosocial changes. During this time, adolescents undergo pubertal changes which are not only physical but are also accompanied by cognitive developments such as a stronger sense of personal identity and a personal system of moral and ethical values. Additionally, they experience increased emotional separation from their parents and a heightened influence of peers and peer acceptance.