At what rank should Soldiers be exempt from organized PT?

Never. combat readiness should be maintained no matter what the rank or job.

Never! Organized PT reveals to the command who is fit and who is not, regardless of rank.

Why would a senior NCO or Officer want to be excused from Organized PT??? Always lead from the front or don't lead at all. I would say after you retire you can stop coming to PT formation. Don't stop PT, just stop coming to formation. That's what I did. 32 years and I was still attending right up to out processing.

At 60.

I did organized PT with my troops until retiring as a Colonel at age 52. It's about leadership!

What? Requirements may decrease with age but a level of fitness (always Considering why it is necessary; Combat makes no considerations for age), should always be required and participation should never be excused. I'm guessing you mean "exempt from PT Formation" so some duties may allow for alternate PT times, but even then it should be an exception based on mission requirements. Rank should have no bearing... although we all know it always does.

After EAS or retirement.

At retirement

Once it's visually obvious their favorite space is the chow hall.

Never. The senior-most servicemembers are the ones who promulgate, and eventually enforce, such things as separation due to physical status. There is no greater hypocrisy than someone creating and enforcing rules and standards from which they are exempt. And this means the standards have to be realistic. I can remember a Master Chief Petty Officer in my naval reserve unit back in the mid-80s who showed his disdain for the PRT standards by doing the run in his dress uniform, with medals --- he always easily passed the test by covering the distance at a brisk walk. When questioned about this, he stated he would wear running gear when the standards were sufficiently tightened up to cause him to break an actual sweat.