Hi John Smith,
All soft materials will soften and break down over time so minimal body impressions that don’t affect comfort or support are certainly normal but this is different from the degree of softening or breakdown that would be considered to be a defect in the material. Latex is the most durable of all the foam materials and it will soften and break down less and more slowly and maintain its original properties longer than other types of foam but no material that is suitable for use in a mattress is immune from some degree of softening or breakdown over time. There is more in post #4 here about the many factors that can affect the durability and useful life of a mattress relative to each person.
Phoenix