The mid-life crisis is somewhat artificial. Professional age brackets, and varied and sundried social expectations coalesce with the ephemeral life-expectancy to create new forms of awareness and their consequences: of having reached “mid life”.
The Pretenders wrote it up succinctly with their song “Middle of the Road”.
Many avoid it.
People do not all hit it at the same time. The first shock is when people realize they feel a changeover from it having been better to get older – growing up, and then it stopped and apparently rather shortly thereafter, especially for women, there are concerns associated with it. Early on this also emerges as the pressures to marry and to start having children. This is not really middle aged stuff but is intimately related to that, and after a couple of free years as a young full grown adult it can seem that way. Of course, with a life expectancy of only 50, just 25 years old can trigger the mid life crisis whereas when 85 or 100 years of age is the expectancy, then it often won’t set in until the 40s.
During the 30s people crest over into having been adult half of their lives, and beyond that lies having been an adult most of the life. It is actually a pretty major change. In the USA and probably some other nations, at age 35 two other major changes occur. 1) People literally cease to be eligible to start professions that only let young adults start. Cops and the military are both examples of those. 2) People’s age group for taxation forms maxes out on the young adult group of ages 18 – 35.
The other reasons people get emotional about whether or not they are in the middle is simply that the whole method of putting off anxiety about the future by reminding oneself “I’m young and have so much life ahead of me; there’s time. Don’t worry,” to “Oh my God, I can’t even do that anymore, and where am I in my life compared to where I want it to be? How do I really feel about that?