This is known as "Head & Hand "Soiling". Although this is normal over long periods of time, we have seen this happening prematurely in some cases especially on areas where men use the sofa or chair the most.
The reason being is that men generally have stronger chemicals in their sweat. In addition, if someone is taking regular medication such as heart, blood or arthritis tablets, these will have a detrimental effect on the coatings.
After speaking with a professional leather repair expert, the mentioned medications contain heavy metals which are harsh on leather and its coatings.
If not cleaned frequently, the only solution will eventually be leather replacement. So the best thing to do is clean the leather sofa with special attention in the areas where the hands and head touch most.
Proper maintenance will surely prolong the life of the leather sofa.