Well, the basic space frame components of the Enterprise
would be nearly forty years old. As would the basic space frame components of the Yorktown
if it came into service at roughly the same time and followed the same refit in the early-2270's. Yorktown
was near Earth, possibly nearing the end of a mission herself and also being retired.
To me, it doesn't seem like they're retiring the class just those ships that were upgraded from earlier configurations.