It's a judgement call. If your carpet has holes or tears, is stained beyond cleaning, has odors that won't go away (like cat urine) or is worn to being thread bare, then you might want to consider replacing it. Age is certainly a factor in determining the condition of the carpet, but not always. Use and abuse trump age most of the time. The quality and features of the carpet are also factors.
Moldy carpet should not be cleaned, it should be replaced. Consider that by the time the carpet has molded, the padding and underlayment have already been damaged and need to be replaced or repaired.
Carpet pads should be replaced periodically but they can last longer with proper care and maintenance. A number of home improvement stores carry replacement supplies.
It depends on why you need your carpet replaced. If it was damaged due to a covered peril then it will be covered. If your carpet is just old and worm due to usual wear and tear then it is not covered.
A carpet guard isn't completely necessary, but helps to keep the carpet looking clean. Carpet guards should typically be replaced about every two years.
For allergy or general cleanliness health concerns the carpet should be replaced with a hard surface like wood or tile.
No, but you should not have ruined the carpet with stains and wear in that time. There should be allowances for normal wear.
Usually if you buy new carpet they will remove the old carpet and recycle it for you for free. IF this is not possible go to your local dump they should be able to recycle it and it will be out of your way.
Some carpet can be repaired easier than others. Most can have a section replaced if you have some of the same carpet or perhaps a piece could be taken from a closet or from under a piece of furniture.
It sounds like their house could use a renovation. Replacing the carpet and repainting the walls would be a good way to freshen up the space and give it a new look. This could increase the aesthetic appeal and potentially even the value of the property.
You should scrape the old glue off if you expect a flat surface.
nope - you do not replace old carpet with new carpet at your cost. PERIOD
Most carpet manufacturers include a "rated for" length of time for the life of the carpet. The most common is 10 years. The price of the carpet usually reflects the life expectation of the carpet. The more expensive carpet will generally last longer. If you have purchased a new house, you can get the carpet information from the builder. It you have purchased a pre-owned house, you should ask the seller when the carpet was last replaced. If the seller replaced it prior to sale, ask for the paperwork, or plan on replacing it within 5 years, since sellers always choose the most inexpensive carpet. Other factors on the life of a carpet are maintenance and wear pattern. For longer life, frequent vacuuming is important, and yearly (at least)professional cleaning.