I don't think your grades expire, it's been about 10 years since I took gen chem but I'm off to med school in August.
I think the issues is: can med schools be confident that your knowledge of the sciences is sufficient for their demanding classes?
Your MCAT does expire, and it is supposed to be an assement of your skills in the sciences. So, I think if you did do well in your prereqs years and, and if you do really well on the MCAT recently, then I think you're good to go. If you do poorly on the MCAT or have a low GPA from years ago, well, then you might have to take more science classes. I would never retake the prereqs unless you really felt rusty
Good luck!