Jesse Karmazin claims he can fight aging, and a lot of people want to believe him. : Jesse Karmazin, the 34-year-old founder of the startup Ambrosia, had a pitch journalists couldn’t resist: For a fee, he could help his clients combat aging and its related ills with infusions of blood plasma from the young. Teen donors, vampiric undertones, a serious-sounding study, an $8,000-per-person price tag and rumors that venture capitalist Peter Thiel might be interested earned Ambrosia more than 100 press mentions in just two years. The effects of young plasma, Karmazin told reporters, were astounding.