Until the carpet becomes a safety or health hazard, landlords likely do not have to replace the carpet if replacing the carpet is not something the landlord is contractually obligated to do.
If you have a written agreement with the landlord, you should review the agreement to see what the agreement's provisions are. Most agreements do not have any provision that landlord must replace carpet after a certain period.
You can talk with the landlord to see if the landlord will replace the carpet for you. If there is a particular type of carpet that you like and you will be living there long enough, perhaps you can suggest to the landlord that you will pay for some of the cost of the new carpet.
Alternatively, if you do not mind moving, the rental market currently is more favorable to tenants than to landlords.
I agree with Mr. Nguyen. There is no rule or law about replacing carpets. They are only going to be replaced when they are so vile that the landlord cannot get the rental amount he expects unless or until he replaces the carpet. Even with no damage, just normal wear and tear, they wear out eventually.
But there is no statutory mandate to replace carpets in any amount of time.