I was like 10 or 11 when I decided. I'd read some book on level 4 viruses like the ebola virus and decided I wanted to become an infectious disease epidemiologist and physician to research these awful diseases and help patients. My mom said no way in hell was she letting her only child go die over in some other country. She told me if I contracted the bugs they'd throw my body in a mass grave and burn it and she'd never see me again.
So.... then I decided I wanted to be a storm chaser. This didn't go over well either for some strange reason
Anyway, got older, decided I should put my insatiable curiosity to good use and thought I'd be a scientist and do research and help people behind the scenes, but still thought about being a doctor quite often. Tracked for a neuroscience PhD, while talking myself out of medicine on the grounds that it was too expensive and that I probably wasn't smart enough.
Wound up leaving my grad program and studying clinical lab sciences, fell in love with everything I studied from hematology to immunology to parasitology. Realized I was stupid to keep talking myself out of medicine. Also fell in love with micro all over again.
Decided to get a job for a few years first, then while working decided to do an MPH in epidemiology as the employer would kick in for it. Focusing on infectious diseases.
So 20 years later.... I'm back to planning on becoming an infectious disease physician and epidemiologist.
Sorry Mom
edit* and as you can see from my profile pic, I didn't listen too well about the storms either