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“Moderate dirt or spotting on a carpet or drapes, even if you can't get the stains out, is probably just ordinary wear and tear if the tenant has occupied the unit for a number of years. On the other hand, you would be justified in deducting from the security deposit for large rips or indelible stains [or cigarette burns] in a carpet. The basic approach to take is to determine whether the tenant has damaged or substantially shortened the life of something that does wear out. If the answer is yes, you may charge the tenant the prorated cost of the item, taking into account how old it was, how long it might have lasted otherwise, and the cost of replacement.”
― David Wayne Brown, The California Landlord's Lawbook: Rights & Responsibilities

“The landlord, however, may not use the security to repair or improve conditions that existed before the tenancy began or that are the cumulative effect of ordinary wear and tear occurring during any one or more tenancies. CC §1950.5(e).”
― Myron Moskovitz, California Eviction Defense Manual