When should I register for an ORCID identifier? I plan on starting my PhD in a few years. Is there any particular time that I should register for an ORCID identifier? Can I do it now (still an undergraduate student), or should I wait for acceptance to a PhD program? Or, am I overcomplicating this and the time of registration doesn't matter?

This may seem like a basic question, but I couldn't find any information on the website.
PhD is not the requirement to get ORCID identifier. You will need it mostly when you are ready to submit your research work in journals. In my area of research (Computer Engineering) most of the journal has made mandatory to have ORCID identifier.