From teenage blood!
https://www.thesun.co.uk/news/4282685/teenage-blood-us-san-francisco-ambrosia-immortality-elderly-mature/
Sounds like a scam, but the company was formed by someone from Stanford, and we have reported/linked to previous research showing important benefits from adding teenage blood to older people. Why, presumably because the telomeres in the young blood are longer, so the stem cells can reproduce for longer periods, etc.