What do you expect from a PhD student before being an advisor for him? I mean do you expect from him to fully understand your papers and to be able to contribute your research from the first day or first month? Or, do you ask him to join your research group meetings and ask him to do some minor tasks in the group (collecting data, maybe some coding) while still learning about your research and other relevant topics he needs to understand the papers.
For example, assume that you are working on manifold learning and your PhD student has some experience on machine learning and has a very superficial knowledge about topology and manifold learning. Do you tell him to go and not to come back till getting some advanced math classes. Or do you rather tell him: "You did well on the general machine learning class which I taught why not come and join my group and lets have a look what we can do"?