Do FDRs & CVRs (black boxes) have an in-service life span?
The FAA is making an effort to phase out magnetic tape style recorders as noted in the this AC 


  1.7 Obsolete Technology. We are phasing out magnetic tapes through
  mandates and voluntary efforts and replacing them with modern
  solid-state recorders. These new recorders not only enhance safety but
  also benefit you directly, as they avoid the high costs and technical
  problems of maintaining outdated recorders. Modern recorders do not
  require the hourly replacement of tapes and calendar weighing of
  thermal capsules. Also, existing tape recorders no longer meet the
  most recent FAA TSO minimum performance standards.


However they clearly state the replacement is voluntary. The wording here also implies that the tapes are replaced on an hourly cycle and the thermal capsules are checked. 

This AC covers quite a bit about the design and function of the FDR and CVR. I would take a look at CHAPTER 3. CONTINUED AIRWORTHINESS REQUIREMENTS starting on page 12. It outlines some of the things old FDR/CVR's may need to comply with to remain airworthy. 

Im sure there are also specific limitations on specific units out there like any other component in aviation that may have caused them to be removed from service or lose their certification somehow.