The results of the Seattle Longitudinal Study show that the ability to react quickly to stimuli decreases measurably from the age of 25. However, it will be decades before this change becomes a noticeable disadvantage, because regardless of the speed of perception, the different thinking skills, especially logical thinking, remain intact into old age. Above all, lifestyle in middle age has a significant influence on how long people remain mentally fit, with chronic stress and little exercise having a negative effect on the brain. Verbal skills are retained somewhat longer than numerical skills, presumably also due to exercise, as few people have to do mental arithmetic all the time in their everyday lives. When the ability to think begins to decline noticeably, it usually does so on all levels, whereby around 50 per cent of the changes can be explained by mental ageing, as these run parallel to physical health, i.e. due to the physiological parallels between the cardiovascular organ and the brain. As the heart and brain require a lot of energy, which they obtain from glucose, this means that the older the organism gets, the less efficient this process is, as the power stations in the individual cells, the mitochondria, are less efficient and the heart and brain become weaker. The older a person gets, the less grey brain matter they have available, but not as a result of dying cells; rather, the synapses are lost over the years, which applies to most people, regardless of whether their ability to think suffers. Those who can still think clearly in old age can compensate for ageing, for example by using other nerve cells.