The correct answer to this open question is the following.
Walter is a college student who is about to graduate from college. The age in which he would most likely consider that a substantial change is occurring in his life is approximately when entering adulthood at 20 to 22 years old.
When a college student passes adolescence into adulthood, he/she realizes the many changes the body is having, he/she is in the process of becoming more responsible and accountable for the decisions and actions it makes, and the painful process of accepting the consequences of its actions and decisions. That is why makes a college student grows and be a mature person.