Boundary construction is most evident in three-year-olds.....

“ Boundary construction is most evident in three-year-olds. Boundary construction is most evident in three-year-olds. By this time, they should have mastered the following tasks:1. The ability to be emotionally attached to others, yet without giving up a sense of self and one‘s freedom to be apart, 2. The ability to say appropriate no's to others without fear of loss of love, 3. The ability to take appropriate no's from others without withdrawing emotionally. Noting these tasks, a friend said half-joking, "They need to learn this by age three? How about by fourty-three?" Yes, these are tall orders but boundary development is essential in the early years of life. ”

Henry Cloud

From : Boundaries: When to Say Yes