[Goose]

Question, can non-athletes who graduate early from high school start classes at Stanford in the spring quarter or do they have to wait until the fall quarter?

Recently Stanford has allowed "early matriculation". I do not believe this opportunity is restricted to athletes, but I suspect few, if any,  non-athletes are going to take advantage of it. Someone here may know of non-athletes that did so.

If only athletes are able to get this benefit, is it too far fetched that they may also get the benefit of applying to a grad program after the "non-athlete deadline"?

The two processes are completely different. Undergraduate admissions is a university wide process and is not controlled by the various "schools of" in the university. It is arguably a fairly formulaic and "bureaucratic" process. The administration can bend the rules fairly easily and can argue that the law of large numbers keeps it fair for all concerned. This hides in large measure athletic preferences. At "larger" schools e.g. ASU, they admit so many undergraduate students almost anything they do is justifiable.

Graduate school admissions are controlled by the schools and their faculty directly. The faculty is jealous of their prerogatives and are generally firmly opposed to anybody who wants to interfere in the process. It is highly unlikely they would countenance a process that involved giving athletes any "special" consideration. Even if some faculty members personally wouldn't object, the optics of granting "unequal" opportunities for any CLEARLY non academic purpose would IMO preclude such an action.

As BC has pointed out, grad school admissions are set up at least in part to grant a necessary certainty and predictability to the applicants. Changing the schedule for everybody else to allow athletes to enter the portal and then apply to grad school is obviously impractical. Making exceptions for athletes would publicly and unequivocally demonstrate that some grad school admissions  aren't about grad school. I can't see that ever happening.  The again, lots of things that used to be "impossible" have happened. I can't imagine this would be such a case however.

[Goose]

Question, can non-athletes who graduate early from high school start classes at Stanford in the spring quarter or do they have to wait until the fall quarter?

Recently Stanford has allowed "early matriculation". I do not believe this opportunity is restricted to athletes, but I suspect few, if any,  non-athletes are going to take advantage of it. Someone here may know of non-athletes that did so.

If only athletes are able to get this benefit, is it too far fetched that they may also get the benefit of applying to a grad program after the "non-athlete deadline"?

The two processes are completely different. Undergraduate admissions is a university wide process and is not controlled by the various "schools of" in the university. It is arguably a fairly formulaic and "bureaucratic" process. The administration can bend the rules fairly easily and can argue that the law of large numbers keeps it fair for all concerned. This hides in large measure athletic preferences. At "larger" schools e.g. ASU, they admit so many undergraduate students almost anything they do is justifiable.

Graduate school admissions are controlled by the schools and their faculty directly. The faculty is jealous of their prerogatives and are generally firmly opposed to anybody who wants to interfere in the process. It is highly unlikely they would countenance a process that involved giving athletes any "special" consideration. Even if some faculty members personally wouldn't object, the optics of granting "unequal" opportunities for any CLEARLY non academic purpose would IMO preclude such an action.

As BC has pointed out, grad school admissions are set up at least in part to grant a necessary certainty and predictability to the applicants. Changing the schedule for everybody else to allow athletes to enter the portal and then apply to grad school is obviously impractical. Making exceptions for athletes would publicly and unequivocally demonstrate that some grad school admissions  aren't about grad school. I can't see that ever happening.  The again, lots of things that used to be "impossible" have happened. I can't imagine this would be such a case however.