Hi supravista,
In North America … compression hardness is measured by a material’s Indentation Force Deflection which is a measure of the firmness of the material. With latex it’s usually called ILD (Indentation Load Deflection) and is measured slightly differently from polyfoam. There is more about ILD/IFD in post #6 here.
All foam materials will soften and break down over time and this results in both the loss of height (decrease thickness) and loss of firmness (ILD/IFD). The foam will soften and the ILD/IFD will become lower under the heavier parts of the body particularly where weight is more concentrated. Mattress warranties cover loss of height (visible impressions in a mattress) that are greater than the warranty exclusion (usually from .75" to 2" or sometimes more depending on the mattress design and the quality or the materials) but they don’t cover the loss of firmness in the materials or “virtual impressions” which can only be felt but not seen. These virtual impressions from foam softening but without significant loss of height leads to the gradual or with lower quality materials the more rapid loss of suitable comfort and/or support in a mattress. This loss of comfort and support is the main reason that most people will need to replace their mattress. There is more about the many variables that can affect the durability and useful life of a mattress relative to each person in post #4 here and the other posts it links to.
Phoenix