Do older Kitchenaid mixer attachments work with a new mixer?
Does your question relate to hub attachments? If yes, then they should fit.
If your question relates to things like bowls, beaters, etc. they tend to be model specific. I don't think, for example a bowl or beater from a 4C will fit any of the newer models.
Your question reminds me how much I miss the old KA Conversations forums. Kitchenaid shut it down a few years ago and moved to a Facebook page. It's a very different environment, but may be helpful to you if pose questions to a KA rep.
I suppose it depends on which mixer, how old, and which attachments. Kitchenaid lists currently available accessories for various models:
As you can see, different models have bowls and beaters of different sizes and shapes, and I wouldn't assume that other attachments are interchangeable either.