Deferring for non-academic reasons I applied to PhD programs for Fall 2018, and am currently evaluating my options. I have narrowed myself down to two options, both good choices with potential advisors who are a good fit, though I prefer one school slightly over the other. 

The reason for my post is that I am interested in deferring my admission for a year. My understanding is that graduate schools typically let students do so when the student has a "good reason", i.e. those who want to spend time studying abroad, doing volunteer work, etc. The catch here is that I don't have any particular opportunities I want to pursue, I just want to take a break and work on some mental health issues I have before signing on for a 5-6 year commitment (really even longer considering the time afterwards spent in post-docs, trying to find tenure-track jobs, etc. if I want to try to stay in academia). 

I haven't heard of any situations like this, and I'm not sure how the graduate schools will react if I ask. Is it possible that they'd revoke my offer? I've also already spoken to potential advisors, should I let them know? I'd be worried what they might think if they knew I wanted to defer graduate school for this reason.
As a university professor I supervise graduate students including international applicants. I'd say that the situation depends on a particular institution, but unless you want to know the regulations inside out, the best thing to do is to build trust with your future supervisor and ask his/her assistance to clarify the case with university administration.

Say, as a supervisor I don't really care whether a student comes this year, next year or in two years. As a human being, I might be curious about the reason, but in most cases it won't be an issue (a year-long break after enduring undergrad studies is a perfectly legit reason for me).

However, there is also bureaucracy. If you got a scholarship, it might be a serious trouble, as it is often tied to a particular academic year, especially if it you are dealing with an external funding organization. The school could initiate a certain formal process that might be difficult to postpone, but you won't find out the details without your supervisor's support.

Say, at our institution we typically have enough room for potential MSc/PhD applicants, and the real issue is their financial support. If a certain applicant is not funded by us and informes us in advance that we should expect him/her next year, it won't be an issue. However, formally we "reset" the selection process every year (so the results of the last year applicant are considered "stale"), so if there is a real competition among participants, it might become a trouble as well.

Sorry for not being very specific, but there are simply too many factors to evaluate, so an insider is your best bet, really.