For many years, the mattress industry stated that the average life of a good mattress set was in the 8-10 year range. Now the industry and the Better Sleep Council recommend replacing your mattress every 5-7 years.
It is difficult to say what the more accurate guideline is because the life of a mattress is influenced by several factors: the quality of the materials used in the product, the weight of the person(s) sleeping on the mattress, whether the person is a side/back/stomach sleeper and how much the person moves while sleeping. Also, unrelated to durability, comfort preferences can change over time so you may sleep better on a harder or softer padded mattress after a few years.
On average, regardless of the mattress brand, a mattress should last 8-10 years at which point it starts to lose its ability to offer proper support. This does not mean that the product has a warranty issue, it just means your mattress is wearing out.