You mentioned "reliable" methods, and I would consider none of the medical tests as measures I would want to rely on. GS's benchmarks make more sense. Mine are:
You wake up and bounce out of bed refreshed.
You work all day, get home and play with your granddaughter, clean the house, and are surprised when it's time for bed and you aren't dead tired...
You enjoy the work-week as much as you enjoy the weekends.
You feel mentally healthy, joyful, enthusiastic, alert, peaceful, patient - never depressed, never anxious.
You wear no make-up, no hair-color, yet people guess your age to be 20 years younger than you actually are.