My wife gets pissy when I refer to myself as "middle aged".
From Wiki (according to this, it's your third quarter of your life, so the first quarter must not count, if the third quarter is the "middle"):
Middle age is the period of life beyond young adulthood but before the onset of old age.
Various attempts have been made to define this age, which is around the third quarter of the average life span of human beings. According to Collins Dictionary, this is "... usually considered to occur approximately between the ages of 40 and 60", The OED gives a similar definition but with a later start point "... the period between youth and old age, about 45 to 60." The US Census lists middle age as including both the age categories 35 to 44 and 45 to 54. Erik Erikson sees it ending a little later and defines middle adulthood as between 40 and 65.