Science likely can not determine exactly when a man progresses from young to old.
But, I can.
Based on my experience, at least.
One day, I had dreams: become Special Forces-qualified, gain a career with the CIA... save the world.
Fantasies of youth.
And then, the next day, dreams were replaced by goals.
These much more realistic in their attainability: a healthy retirement, a house, a dog; eventually, return to North Dakota.
Am I old?
Old enough to value the pursuit of goals instead of dreams.