Hi mrsrats,
I split your post into a new topic of its own to keep the original thread from straying into a new subject 
If your mattress is still in very good condition and there are no sags or soft spots in the sleeping surface and you just need some additional softness and pressure relief then a topper can certainly be a good idea and can extend the useful life of the mattress. If you decide to go in this direction then post #2 here and the topper guidelines it links to can help you to choose a topper that would have the best chance of success.
Most pillowtops that you will see in the mainstream industry or that are made by major brands use lower quality/density and less durable materials in the pillowtop which is the biggest reason that they tend to soften and break down relatively quickly. The quality/durability of the materials in the pillowtop will have more to do with how long they last than the fact that a mattress has a pillowtop although it’s also true that thicker layers of softer materials in the upper layers of a mattress will also tend to be less durable than firmer materials or the same type. In other words the useful life of any mattress depends more on the quality and durability of the upper layers of the mattress than on any other single factor. Latex (which is the most durable foam material) along with higher density memory foam or polyfoam in a pillowtop or the comfort layers of a non pillowtop mattress will all last for much longer than lower quality and less durable materials no matter what type of design (tight top, pillowtop, eurotop) the mattress uses.
This is why it’s so important to know the quality of the materials in a mattress (see the guidelines here) so that you can make sure a mattress has no obvious weak links (in the upper layers especially) that can affect and shorten the useful life of the mattress.
This is also the reason that it’s usually better to deal with smaller independent manufacturers (like Fox Mattress) that either sell factory direct or through better sleep shops because they tend to use higher quality and more durable materials in their mattresses in every budget range than the major manufacturers.
A mattress that lasts for 15 years and is still in good condition would be almost unheard of in the mainstream industry or with major manufacturers today.
Phoenix