Just think if you did not have a birth certificate nor anybody who remembered when you were born. What age would you give yourself? Science is interested in this question. Not everyone feels the years in the same way. Some feel older than what they are and some younger and Medicine now speaks of Subjective Age. What we will do tomorrow or within a year very much depends on subjective age. Many health- related issues and our ‘risk of death’ depend on the age we believe we are rather than chronological age. Research would like to know if there is a point of intervention to feel younger. People who feel younger than what they are, run less risk of suffering from various diseases such as dementia or depression and are less likely to be hospitalized. In a study on 17000 people over 50 years of age, feeling that you were 8-13 years older than what was the case carried a risk of death that was increased by 20-25%. Generally, in childhood and adolescence one feels older than what actually is the case. At 25 one feels one’s age and then progressively younger until 50 years of age. From then on two populations form, the one continuing to feel younger and the other older than true age. If instead the question is: “How old would you like to be?”, the reply coincides with the age you feel you are only for those who feel younger. Instead, for those who feel older the age given is not only younger than the one they feel they are but also less than true age. Researchers believe that the difference between the two populations is that the former are optimists (the age they think they are is also the one they would like to be and both are lower than true age. The latter are pessimists (the age they think they are is much older than the one they would like to be and the two ages are on opposite ends of true age). In brief, seeing the glass as half full facilitates filling it and seeing it as half empty facilitates its emptying.