In most modern nations, "adult" refers to someone over 18 to 21. With life expectancy of over 70 years, that places middle age exactly in the middle of 50-year period. I thought that was obvious enough.
Hey if we're lucky maybe Ray Kurzweil's Technological Singularity will set us free. :p
Post-college working life: 22 - 72 (extended SS eligibility age for the USA). Half of that: 47.