Many pressed CD-ROMs and audio CDs will likely last over 100 years if stored properly. However, the tolerances in the standard are high so lots of manufacturers use cheaper materials and processes than intended while still producing discs with the CD logo. It will take decades before we really know which were worse than others. The most likely point of failure is delamination, which will leak air between the layers and oxidize the aluminum coating in a process named laser rot after Laserdiscs, the earliest commercial optical disc system.

Anyway, the longetivity of (re)writable CDs is indeed usually below 20 years.