I am a law school graduate. What I offer is mere information, not to be construed as forming an attorney client relationship.
1. Normal wear and tear is considered the normal erosion/abuse carpet takes when a person inhabits an apartment. perhaps a little dirt, frayed edges due to walking and indentations where furniture was. Spots are usually considered wear and tear because when the professional carpet cleaners come in (after you move out) to clean, they can usually get rid of it with spot remover. Stains that require actually patch replacement of carpet is NOT considered normal wear and tear.
2. A tenant is usually responsible for replacement of carpet when there are stains, huge tearing, anything REALLY BAD -- like burned carpet, etc. What is not covered from your security deposit, will be billed to you.
3. Age of carpet is sometimes a factor in replacement cost. If the carpet is 20 years old, chances are you wouldn't be responsible. If the carpet was 5 years old, get out your check book. So maybe pro-rata charge for the 20 year old carpet. "When is the tenant responsible for the full price of the carpet replacement?" I just answered this essentially.
Hope this helps.