The '5 year clock rule', only applies to NCAA Division I athletics per my understanding (I will say that the rules are constantly changing by the way).
In general, the 5 year clock rule says that you have 5 years to play 4 seasons of competition. This allows for a redshirt year. In NCAA DI, once you start your clock, it runs without stopping (exception for military or religious service) for 5 years and within that 5 years you have 4 seasons of competition.
A student-athelte starts their clock when they enroll full time in a college, participate in practice or competition with a team, or play in a competitive season with another sports/organization entity.
A newer DI rule now automatically starts a player's clock 1 year after the first opportunity to enroll in a college after high school. So, if a player graduates June of 2013, the player's clock would automatically start August 1014. The NCAA gives an athlete a 1 year grace period, then starts the clock.
For NCAA DII, there is the 10 semester rules. This says that a DII player has 10 semesters to play 8 semesters of competition but the semesters do not need to be consecutive. This provides a bit more flexibility for what life can throw our way. I am not sure if DII has an auto start to their clock after 1 year post high school graduation.
For DIII, I am not aware of their eligibility parameters.
NAIA is known for having a much more open eligibility stance when it comes to age. They also allow four years of competition, but I am not sure about when they start their clock and if it runs in consecutive years. I do know it is quite common for older players (mid to late 20's) to be competing on NAIA teams.
Because of how 'thin' that answer was, I would ask that any of my VolleyReaders who are DII, DIII and NAIA coaches, please share your knowledge in the Comments section of this post, so all readers can see!