They say you're only as old as your arteries. Then again, you'd need to be able to ask them. But this is what more and more American startups are offering: For an online payment of $200-300, some companies will send you a kit to collect a sample of saliva, urine or a few drops of blood. A few weeks later, you will find out your biological age based on the health of your cells.

These tests bring joy to longevity obsessives, who are already the target of all kinds of books, podcasts and dietary supplements. In the quantified self era, they already have an array of tools. It's understandable that once you've tried intermittent fasting, a keto diet, tracked your steps, calculated your hours of sleep and taken various pills, you'd want to measure the results, in the same way as WeightWatchers members want to step on the scale. The aim, of course, is to achieve a biological age well below your actual age. Some people even post their results on social media, just as others show off their pectoral muscles. It's as if to say, the inside is just as fabulous as the outside.

Most of these tests claim to use an epigenetic clock created by comparing changes in DNA methylation with those of a large sample of people of all ages. However, as they do not all use the same clocks and methods, they do not give the same results. That's why Suresh Nirody, a former healthcare executive in Cincinnati, Ohio, underwent testing by several of these companies.

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Elysium biological age test put Nirody's biological age at 58, but his brain age at 73. "It was a shock! I immediately made a point of sleeping better and exercising more." More importantly, he immediately searched the internet for references to other test brands. He had already received the results from Thorne, which put his biological age at 69 years and nine months and his kidneys at 74 years and eight months. To understand what to make of it, he is waiting for figures from a third test, myDNAge. Until then, he is still 64, or somewhere between 40 and 80 like most people who get tested, if Robert Brooke, co-founder of Clock Foundation, an epigenetic age site that sells kits on myagingtests.com, is to be believed.

Brooke isn't surprised by the number of people testing their age with kits from different companies. "In fact, they want to test the very idea of testing." He wouldn't provide Le Monde with his biological age; he just said he was "a little younger" than his actual age. "Unfortunately not dramatically younger," meaning his result fell within the average range of people who test themselves (which isn't surprising since longevity obsessives are more likely to have already adopted a favorable lifestyle). However, he insisted that the result was of little importance in itself, "the point is to do it regularly to measure the impact of a remedy you're taking, for example."

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