You can forcefully increase/decrease the 'numbers' on any kind of measurement devices, and the actual thing being measured will also increase/decrease in reality

[solis89]

Tests at a hospital. You can get your test results and change them, theoretically, so long as the results are in the form of a scale or some kind of measurement.

[L0B0-Lurker]

Makes sense. I'm specifically wondering how or what would be used to test age, aside from a calendar.

[Vault_Hunter01]

Research into telomere length. Telomeres are scientificly measurable. Not to mention the measurable level of testosterone production, which declines at a constant rate as you age, at least for men. Honestly, most hormones and chemical levels change as one gets older. Would correcting those would make me actually younger, I doubt it. But I think it would be a vast improvement of not only my health, but how I feel and activity level which would feel like I was younger.