There are critical periods in development when the person is more vulnerable to harm or available for growth. Just as the fetus is more vulnerable to harm and also develops exponentially during the first trimester, there are other critical periods for different kinds of development. Expectant mothers risk harm to their unborn children from infectious illnesses earlier in gestation that become relatively benign in late pregnancy. Early childhood is clearly a critical period for second-language acquisition. Teachers of preschool and early elementary school consistently experience amazing English language acquisition and proficiency within a school year among their non-English speaking students. In contrast, for many people second-language acquisition can be an arduous process despite when as adults, they are at their most advanced intellectual development. While a second-language can be acquired and relative proficiency achieved, it often takes much more effort and time because adulthood is not the critical period for easy second-language acquisition. This seems true for all save for some linguistically gifted individuals.
The athletic challenge of learning to ride a two-wheel bicycle seems easiest in early elementary school age as opposed to the non-critical period of adolescence or adulthood. My siblings and I didn't have bicycles during my childhood. And I didn't get the opportunity to learn to ride on neighborhood kids' bicycles. While I could and did learn how to ride a bicycle in my late teens, it was more awkward and never as fluid of a process for me as it seemed for the young 7-13 year-old daredevils charging about the city. They had started and became proficient riding much earlier. Of significant concern for therapists treating individuals, couples, and families is the critical period for forming attachment with primary intimate individuals from birth throughout childhood, specifically with parental caregivers. As with all critical periods for development, healthy or successful resolution portends more adaptive and functional results in future life experiences. Conversely, insufficient or problematic attachment development can have significant adverse effects on later functioning. An individual's adult relationship is predicted by early childhood critical periods around attachment and other development. A challenge for the individual, couples, and families and for therapy is that negative or inadequate development during some critical periods may have disabled or corrupted healthy functional relationship development.
When the individual, couple, or family comes for therapy, the individual, partners, or families often have already manifested problematic dynamics due to developmental issues no one may be fully conscious of. One, both, or more individuals may have reacted poorly in the critical periods of their relationship. Therapy may be an attempt to compensate for inadequacies meeting mutual needs in a relationship's critical periods. The process of therapy itself can be considered a developmental process with critical periods as well. Initial contact and the first session or sessions of therapy constitute critical periods to develop rapport, credibility, and contracting the therapeutic relationship (for an extensive discussion on this, refer to my e-book "Therapy Interruptus and Clinical Practice, Building Client Investment from First Contact through the First Session"). Developing rapport can be a major dilemma if the individual client or one or both partners, or one or more family members already have insecure attachment styles that can result in difficulties attaching to the therapist. If the therapist does not develop rapport/attachment between self and the clients in the critical period of the first session or early sessions, it may be very challenging to recover from the failure. Therapy may become a frustrating replication of poor attachment: childhood insecure attachment with caregivers, insecure relationship (couple and family) attachment, and insecure therapist-client attachment.