First of all, once you leave a job that requires a clearance, you lose access to classified information. There's no retirement clause that lets you keep access.

You can, within two years, have your access reactivated without a new investigation if you take another job requiring a clearance.

Executive Order #12968, dated Augusr 4, 1995, section 3.3.3.d states:

Access eligibility shall be reapproved for individuals who were determined to be eligible based on a favorable adjudication of an investigation completed within the prior 5 years and who have been retired or otherwise separated from United States Government employment for not more than 2 years; provided there is no indication the individual may no longer satisfy the standards of this order, the individual certifies in writing that there has been no change in the relevant information provided by the individual for the last background investigation, and an appropriate record check reveals no unfavorable information.

In other words, since 1995, your clearance is still active for 2 years, or until your periodic investigation expires. But you can't actually access classified information unless you both have a job requiring access and have "need to know".