Absolutely at any time you decide you want to jump into a career in UX Design. It makes sense to get someone in the beginning if you are absolutely clueless on how to start out so you will understand what needs to be done to become a UX Designer. A mentor will be able to guide you on how you can begin working on your skills, what you need to learn, how to learn and how to gain practical experience. You could also get a mentor when you have started picking up skills are looking to gain some practical exposure and knowledge on how things work.
Looking for a mentor at various stages of your UX Design journey also depends on the types of mentors out there; some of them can be really helpful and provide valuable insights into how you can make a mark in the field; while some will expect you to at least have a basic understanding of what UX is all about. It depends on what kind of mentor you approach. Be sure to get in touch with someone who is knowledgeable, experienced and is aware of whatever that keeps happening in the field of UX.