Ideally 22/23, I'm starting to think that if I'd got into medicine at 18/19. I probably would have regretted it.
I don't think most hopeful medics (even medics themselves) know what they're getting themselves into, though people see the downsides to medicine, they try to void it out (what I do) and reassure themselves saying 'they'll be able to handle it'. The 'glossy front page' of medicine is what appeals to most people. If they say otherwise, they lie.
You just have to actually become a doctor to know whether you love/like it or not. All the experience in the world is still insufficient.
A lot of the doctors I met weren't raving mad about medicine nor were they extremely passionate about it. Some were kind of indifferent to the fact that they were junior docs and some sounded as if they regretted picking medicine instead of say, dentistry. I personally couldn't do a job I wasn't passionate about.