Under Medicare secondary-payer rules, is there an age limit for actives when Medicare becomes primary?
I have an 80 year old actively employed and benefits eligible

no, see article listed on May 5 @ benefitslink -

Coordination of Benefits: What Plan Counsel Should Know about Medicare Secondary Payer Compliance (PDF)
9 pages. Excerpt: "The Medicare secondary payer ('MSP') coordination of benefits ('COB') law is codified at 42 U.S.C.A. § 1395y (West Supp. 2005) and implemented by regulations at 42 C.F.R. Part 411 (2005). Stated simply, it prohibits a group health plan through which health insurance coverage is provided for employees and their dependents from 'taking into account' the fact that an individual who has current employment status is entitled to Medicare benefits." (Powell Goldstein LLP via Blue Cross and Blue Shield Association)