I would agree with the spirit of the above responses, but wanted to add a few comments, particularly about light exercise.
Would it not be better to look at T'ai Chi as a practice involving forming postures rather than achieving postures? In other words, I believe T'ai Chi focuses on processes rather than directly on results. Think of lowering and raising, rather than being low or being high. Think of emptying and filling rather than being full and empty. Suspending from above is an active thing and not the same as having an erect posture, which is static.
Mere walking involves putting the entire weight of the body onto one leg and maintaining a certain amount of balance. I am not sure that Golden Rooster Stands on One Leg need be any different in essence.
Once one shifts as much weight as possible onto one leg and begins to point the knee and toes of the other leg, I think the core of the posture is there. One does more only for clarity in the feelings and for physical training. Viewed in this way, I would call this light exercise.
I have practiced with one or two individuals who could walk, but could not maintain balance on one leg and therefore could not hold any kicks, no matter how low they kept their feet. Generally, they would flick their feet one or two feet off the ground and then would be forced to step down to avoid toppling over.
Although this has the appearance of doing a T'ai Chi kick and then losing one's balance (I am certainly familiar with the latter circumstance), I believe it is not really the case. I believe this practice involves double weighting, no feeling of suspension from above, and improper focus on balance from beginning to end of the movement. Despite the movement, it is essentially an alternation of jerky and stagnant movement.
I believe individuals in this circumstance are best served by merely lightening the weight in the "kicking" foot, rather than lifting it completely off the ground. By lightening the pressure on the foot, they can shift weight, focus intent, and maintain continuity.
As for the arms, there are certainly individuals who cannot lift them over their heads, but this surely does not characterize most elderly. I think of T'ai Chi arm motions as being like cultivating the feelings one experiences in visualizing drawing a bow and holding it at full extension. The stiffness of the bow, the speed with which one draws it, and how long the end point is held are secondary issues.
During the movement and while holding the bow bent, one can feel the joints open, the upper back protrude, the chest sink, the elbows align, the fingers extend, etc. A stronger bow and a slower speed allow for clearer feelings, but they are not strictly necessary.
Lastly, I think that T'ai Chi is fundamentally about relative states and never about absolute ones. Since this involves a difficult mode of thinking and a difficult mode of expression, I think practitioners do not emphasize this or often forget it. A close reading of much of what the Yangs have written shows numerous places where they shy away from absolute descriptions, but instead employ relative and subjective ones. In this light, working within one's limitations is not merely an option, but a core principle.
Similar to this, the postures are about energy vectors, not positions. The positions result from the interplay of the vectors. Again, think of the bow. One does not preplan the end position or burn it into muscle memory. The interplay of how one opens the joints (as dictated by your intent) determines the final posture. Even in the final position, all the joints from fingertip to fingertip are trying to move. Roll Back, Single Whip, etc. have more complex geometry, but are otherwise no different. Having a rigid posture in mind, rather than letting the energies mix to produce a natural result, involves a lack of "song" (looseness/relaxation).
Copy your teacher's intent as revealed through his or her posturing. Do not copy any of the static positions. This is not an issue of stillness or movement, because one can be static in motion or be actively posturing while still. If you put your intent on copying static positions, you will ignore T'ai Chi principles to achieve your goal.
Most seniors have the physical ability to do this. I would say that if you can walk and comb your hair, you can more or less do standard T'ai Chi. Many people, however, do not have the patience to think and practice in this way and settle for building up a repertoire of static positions that they try to link up in ways that appear soft, smooth, and graceful.
I personally usually lose some of the mental battle between adhering to principle and trying to look good when it comes to the kicks and Squatting Single Whip. Nevertheless, I feel I am ahead of the game by knowing that I am cheating a little.
[This message has been edited by Audi (edited 01-12-2002).]