Chief Justice Rehnquist:the case for compulsory retirement?

Jack and I have been carrying on an e-mail discussion of Chief Justice Rehnquist's health, his competence, and the case for compulsory retirement. Jack began by sending me this [I apologize that I did not keep the specific source and link, but it ran in many papers and on-line news services]:

WASHINGTON - Chief Justice William Rehnquist, sick with cancer but determined to remain at the helm of the Supreme Court, returned to work Friday after defiantly squelching retirement speculation with a pledge to stay as long as his health allows. The 80-year-old chief justice, who is battling thyroid cancer, looked pale but confident as he left his house in a wheelchair for the trip to his office. He spent two nights in the hospital with a fever earlier in the week, and was discharged on Thursday.

Jack then added, after prodding from me:

It's clear that the man has much of his energy diverted to health issues. Only a supreme egotist would presume to carry on such an important job under such circumstance. Age per se is most often irrelevant.

BenS then weighed in with:

I agree with Jack. The guy is mostly disabled, qualified only to teach sociology at this point.

I agree that age per se should generally be irrelevant. But I have some questions about what the criteria for removal from the bench should be if not "life or good behavior."

If age is used, what is an appropriate age for retirement?

If length and number of terms are used, what are the appropriate length and number of terms?

If health is used, you can bet there will be highly politicized debates about whether someone is still healthy enough to serve on the bench.

Whether the criteria for removal/retirement should be changed may be debatable, but if they are changed, I hope the criteria are easily objectifiable, to reduce the otherwise inevitable political debates.

In the end, I really doubt the criteria will be changed. Doing so would require a constitutional amendment. Meanwhile, Congress still has the constitutional right to impeach a judge who is unable to perform his duties but who refuses to step down.