There are two reasons for leather getting hard: age and dryness. Without regular care leather will get stiffer and stiffer with age. Another reason can be shrinking due to heat. The leather in older cars was dyed vegetatively (backside of the leather is brown) and started to shrink at temperatures around 70°C. Without regular care the leather's shrinking temperature is further decreasing. Modern leather has a "Chrome" tanning process and the lifespan of leather was increased due to the prevention of rotting.
A stiff leather can be softened again to a certain extent. The shrinking process of leather, however, is permanent and irreversible. In both cases a treatment with Leather Softener is advisable, as further shrinking of leather can be stopped and stiff leathers can be rejuvenated. Leather Softener is a purely synthetic oil and will neither smell nor oxidise as animal fats or oils do (e.g. fish oil or neats foot oil).