Actually, I have to correct myself, Porsche states that tires over
6 years should be replaced.
Also in light of all of this there are those that will mount tires made by non-Porsche approved manufacturers and be very happy and safe with the results. Goodyear makes a tire: Goodyear Eagle F1 GS-D3, that I understand works well on our cars but is not Porsche approved.
For those interested this is a great link, a tire shoot out! Great info too!
http://www.caranddriver.com/article.asp?section_id=4&article_id=10252