The NCAA five-year clock describes the period of a student athlete's eligibility for NCAA Division I sports (e.g., basketball, baseball and American football). Generally, the five-year clock begins as soon as an athlete enrolls in a college or university. From that point on, the athlete has five years in which he or she can play four seasons (as a freshman, sophomore, junior and senior). During those five years, the clock continuously runs, even if the athlete decides to accept the "NCAA Redshirt" status for one year.
If a student-athlete does not enroll in a university/college immediately following his/her high school graduation, different age restrictions apply to athletes, depending on the sport. In general, NCAA allows a one-year transition period for Division I athletes after high school graduation. After that transition period has passed, the five-year clock automatically begins, in which the athlete is eligible to play for four years.
This applies to all NCAA Division I sports. However, there are exceptions in the sports of ice hockey, skiing and tennis respectively. In tennis, eligibility for Division I athletes already starts six months after graduation. For ice hockey and skiing, the five-year clock only starts after the athlete's 21st birthday.
Sources
Am I too old to play college sports? - Athletic Scholarships