I too saw a few posts stating that there were expirations on pre-reqs. I began to look for any mention of these rules in med school info, websites, prospectuses, etc. I have only looked at the schools I applied to, but I have found no evidence that schools consider pre-reqs to have a shelf-life. I finished undergrad (gasp) 12 years ago. I did a PhD after it, which I finished 7 years ago so any classes I took as pre-reqs are at least 7 years old for bio and some chem and 13 years old for calc and physics. So far, I have not had a single school tell me that my classes were unacceptable. Furthermore, I have an interview and have received several secondaries from schools screen pre-sec. If someone can point to some evidence that shows that schools require classes to have been taken within some time period prior to application, I would be very interested in seeing it.