With the caveat that everyone reacts to altitude differently, in general, many folks who have altitude issues on the Whitney Main Trail (if they are going to have altitude problems at all), usually begin having them somewhere between Trail Camp (~12,000') and Trail Crest (13,600'), as they are ascending the 97 Switchbacks. There are always exceptions, of course; but this generalization seems to fit what happens for many people who do encounter altitude problems on that trail.
CaT
