How do you calculate a seven year carpet against fare ware and tear?

The carpet was two year old when the tenant moved in, but now it’s seven years old and has some stains and tears that need to be addressed. It’s been said that a carpet that old needs to be replace by the landlord at his expense. Does that mean the tenant is off the hook, or can the tenant be charged for a damaged carpet?
Or what would you charge the tenant against his security deposit to replace the carpet when he moves out?

How often does carpet need to be replaced in apartments in Illinois?

We have lived in the same apartment for 3 years in Schaumburg, Illinois and we will be moving out in November. The carpet is very light and after 3 years it has picked up so many stains and discolorations that will not come out. We don’t want to lose our security deposit over the carpet having to be replaced. If you have an answer for me please source where you found it so I can use that. The management at the complex are not honest with anyone and I will need to have something to back this up with.