Measure
Where you are now, in numbers you can actually get hold of.
Why measure anything
Advice that fits everybody fits nobody in particular. A number tells you which half of the advice is for you, and a line of them over a year tells you whether what you changed did anything. That is the only reason any of this is here.
A reference range is not a diagnosis, and the middle of one is not health. Outside a range is a reason to talk to your doctor.
Calculators
Published formulas run on your own numbers, not questionnaires that hand your answers back to you converted into years. Biological age from a blood panel you have probably already had, and more coming.
Blood values
What each value on a lab report is, what it predicts, and where our threshold for it comes from. Every number traced to the study it came from rather than to a guideline.
What you can measure yourself
Grip, waist, blood pressure, resting heart rate. Things that come off your body rather than out of a tube, most of them needing nothing you do not already own.