I just reenlisted in the Army Reserve after a long absence. I do not have a CAC yet. Can I wear ACUs (last uniform I was issued)?

We are in a transition period right now and you can wear the ACU pattern (I think it's actually called the UCP pattern) until 1 OCT 19. It seems as though we're about a 50/50 split between new and old uniforms right now.

When you in-process getting your CAC is part of the process!

ACUs are authorized, but you should get the OCP uniform as soon as possible. It is quickly becoming mostly pending retirees still wearing the old ACUs. You have plenty of time before the wear out date though, as others have noted.