Here is the rule as it is state on the NCAA web site under the frequently asked questions section. I was too lazy to find out which bylaw it is.
"21 Year Age Rule for Division I
In Division I only, if a student-athlete has participated as an individual or as a team representative in organized sports competition, that kind of participation during each 12-month period after his/her 21st birthday and prior to initial full-time collegiate enrollment will count as one year of varsity competition in that sport. Any participation in organized competition during time spent in the U.S. armed services will be excepted.
You may wish to consult with the student-athlete's current college's athletics director to determine exactly how many seasons of varsity competition he/she may have used. Please contact the NCAA membership services staff at 317/917-6222 should you have further questions.