Here is an excerpt from Kodak's FAQ pages for their Ultima CD-R media:
- Kodak claims that the data lifetime on its CD-R Ultima media is 100+
years. How can this be when I have I read that unwritten CD-R media has a
shelf life of 5-10 years?
Because that's as long as unwritten CD-R media has been tested. There is no
reason to believe that unwritten media won't last as long as written media.
Archival lifetime testing is typically performed on written media.
This was written in the early 2000's. Today, by the same logic we can conclude that unwritten, high quality CD-R media has a shelf life of at least 25 years. Basically it doesn't make a difference if the media is recorded or not, unwritten media lasts exactly as long as written media, or in other words there is no indication that the burning process suffers from degradation more than the process of reading the disc.
In my personal experience, with the notable exception of a few low quality cyanine CD-R's made in late 1990s, CD-R degradation usually has been of the reflective layer and not the dye itself. In the burning process reflective layer matters a bit less than when reading the disc since you are altering the dye, which has not degraded in this hypothesis. That's my rationale behind why burning an old disc, even with the usual signs of degradation, shouldn't be a problem.