Is there a law requiring apartment complexes to replace the carpet in the apartment if they know there have been pets? I was wondering this because I know some people have pet allergies and even if the carpet has been cleansed, you can't always get all the allergins out.

If the apartment complex charged the previous renters a pet fee, then that money should have gone to carpet replacement. If not, the previous renters should have been responsible for replacing the carpet. It is crap that you or someone you know got stuck with carpeting that was run down by animals. Has anyone talked to the management at the complex?

A. The pet fee normally does not cover carpet replacement. At the last spot I rented, it was $300. It will cover shampooing, flea treatment, etc.
B. If you are seriously allergic to cats or dogs, do NOT rent at a place that is "pet friendly". Most likely there will have been pets in the apartment before you, but also you are going to be surrounded by people with pets, which means meeting them in the common areas.

No pet fees does not cover replaced, it would cover extreme damage or a good cleaning.

But there is really no laws ( except in mold) that first a place has to have carpet, they can pull up old carpet and have plywood on the floor if they want to, And no there is no law that old carpet has to be replaced at any certain point and time.

You have the chioce to rent or not rent at a place, if you demand new carpet, then have it written into the lease that new carpet will be installed before you move in.