steve horvath of ucla is a biostatistician who's invented an intriguing tool that could help us find a way to roll back the biological clock. steve created a blood test to determine biological age. it's widely considered to be the most accurate in the world. so dr. horvath why is it important to have a test like that? - imagine we want to prove that this is the fountain of youth, right? somebody would have to dive into here. we do a before and after measurement, you know. we would want to convince ourselves that it actually rejuvenated the epigenetic age of that person. - so it's really useful to have this clock as the metric to use if you're thinking of a treatment. - it's essential. a test for biological aging is really quintessential in order to distinguish snake oil from therapies that actually have an effect. - [maren] dr. horvath's blood test measures something called dna methylation. methylation is a process that regulates how genes function. methyl groups get added to dna as we age kind of like rust on metal. using his test, steve has worked out the average rate that this meth