If you’ve heard of billionaire Bryan Johnson, you’ve likely encountered the phrase “bio age testing.” Johnson, the subject of the Netflix documentary Don’t Die, says he’s on a quest to achieve the lowest possible “biological age” (also known as bio age). He reportedly spends millions each year on anti-aging treatments—dozens of daily supplements, weekly acid peels for his skin, injections to boost collagen production, hyperbaric oxygen therapy, and plasma infusions extracted from his 17-year-old son’s body. To determine what’s working, Johnson has undergone numerous tests to assess his bio age. Johnson says these tests show his age is now…