0 to 30 is young
30 to 60 is middle age
60 to 90 is old
^This.
Middle age
begins at 30. Evidence:
1) by age 28 the human cardiovascular system has lost 50% of it's efficiency compared it's peak in youth (no surprise then that most professional athletes are washed up by age 35-37).
2) most people see their first few gray hairs by age 30.
3) Thirty is halfway to 60 and 60 is when old age
begins because almost all reproduction has ceased. Women have gone through menopause. Men are deep into andropause and have less than half the testosterone they had at their peak in their mid-20s.
We have a shelf life but that's okay. Or as Shakespeare said it:
Like as the waves make towards the pebbled shore
So do our minutes hasten to their end
Each changing place with that which goes before
In sequent toil all forwards do contend.
Time doth transfix the flourish set on youth,
Delves the parallels in beauty's brow,
Feeds on the rarities of nature's truth
And nothing stands but for his scythe to mow.
And yet to times in hope my verse shall stand
Praising thy worth despite his cruel hand.