I am a nerd about data. One thing I constantly do is take different tests which measure different health metrics. One which has been very scientifically validated is the World Fitness Test. It is a questionnaire that uses an algorithm to calculate VO2Max so you can take it online. While I prefer the actual VO2Max test done in the lab, these algorithm results are similar to actual, so their science is good. I am 55 years old biologically and have the fitness level of a 37-year-old.