I had seeds planted in childhood, but it really bore fruit in my early 20s I guess.
My real lifelong foundational fetish was tickling feet. And the idea of people being tied up, trapped, immobilized, for the purpose of tickling their feet without them being able to get away. I also had a fascination with embarrassing moments, humiliating people with unwanted public nudity or semi-nudity.
I realized that in medical situations, people are sometimes both forced to be naked when they might not want to be, and also sometimes strapped down or otherwise restrained for their own good. I imagined going to visit a friend in the hospital, finding her in traction and unable to move her feet, and tickling her helpless tootsies to my heart's content. There's also a pretty high chance she might be naked and covered only by a loose hospital drape, which might fall off from all that tickling, or a gown that doesn't *quite* cover everything. I eventually became obsessed with the idea that you could use paralytic drugs to render a patient unable to move but still able be tickled and laugh.
I spent my whole teens thinking almost exclusively about these things almost every time I sexually fantasized. I am *still* really into stories and fantasies about things like this. But all of this is different from what most people call "medfet". It's more like "tickling in a hospital". So I am not sure it counts.
Medfet per se came when I was in my 20s, and had been reading tickling stories online, on USENET and on early tickle websites. Because my tickling interest already had a medical "flavor" to it, I naturally moved over to reading stories about other medical topics that seemed interesting, such as rectal temperature taking, enemas, general purpose embarrassment and humiliation through unwanted nudity.
My late 20s is when I started to get interested in online play exams, and started doing them and receiving them from others. Then in my 30s when I got into BDSM I started doing real life play exams as form of it. Using the humiliation of being forced to wear a hospital gown, medical restraints, the infantilization of a rectal temp, the invasiveness of a digital rectal exam, as tools to exert dominance and control over my partners. I did not ask my partners to call me "Master" or "Daddy", I preferred "Doctor". (And then when they're good and strapped down, and they least expect it, tickle their feet!)