We rent our house in Worthing out to a couple with a toddler and a dog.
It has an old carpet in one of the bedrooms.
I DO expect it to look older a year later - especially with a dog and a toddler.
So, unless it has a 3 foot round burn hole in the centre of it, I would not charge them anything for any 'damage' or anything - and I expect some normal wear and tear.
Even WITH a 3 foot burn hole, I would not charge them the full price of replacing it, as it was already old.
The thing is, as a private landlord, you decide those things based on your own views.
I suspect agencies have set policies, maybe?