We cannot imagine nowadays that a videogame doesn't have an age rating on it. In Europe, we have the PEGI system, in USA we have the ESRB system. In the times of CD-i, other authorities were active more on age rating systems, like ELSPA and BBFC. However, not all CD-i games have an age rating. Apparently it was not an obligation back then to have an age rating logo on the boxart. On some early USA CD-i titles, there is actually a ESRB logo, like on the boxart of The Apprentice:
But other than that, I haven't found a lot of those. Games like Voyeur, The 7th Guest and Burn:Cycle all got a BBFC rating on the box, but the first versions of The 7th Guest and Voyeur (dated 1993 and before that) did not have the logo on the box.
Around 1995, ELSPA was getting more attention and some titles on CD-i actually have a large ELSPA age rating on it, like on the Lost Eden box:
Apparently they also prepared stickers to rate older titles that originally didn't get an age rating, like we see here on an original Litil Divil box:
After 1996, I found a few final CD-i titles that actually received a USK rating, like Brain Dead 13 and Creature Shock (1997):
But most of the CD-i titles didn't have any form of age rating, even newer titles like Atlantis: The Last Resort or The Lost Ride!