You cannot boot from a CD on certain older Macs. I remember reading the list at some point in the early 90s, and I remember the Classic was on there. I'm pretty sure the list includes all the 68000 chip compacts, 32-bit dirty Mac IIs, and the SE/30. I think the IIci is the oldest Mac bootable from CD-ROM.

The "lowest end" Macs bootable from CD are the LC and Classic II.