Surely the slides were of men of an advanced age, right? Otherwise that statement is false.
The question is whether your body sees and kills the cancer cells; most of the time it does, which is why most of us can live pleasantly for most of our lives despite errors cropping up silently. What we term cancer are the cancer cells that slipped past our body's defences, growing into masses that cause no end of grief.
To put it in computer terms, it's like error correction in HDDs and SSDs and ethernet connections. Errors are inevitably going to occur while data is in transit, but they are of no concern so long as error correction and other such mechanisms can correct and recover. The errors do become a concern when they start slipping past such mechanisms, however.
That is as much a philosophical question as it is biological, and one where the answer will vary by who is asked.
Personally, I will say this: If you're miserable while endeavouring to prevent cancer (or any other disease), that's putting the cart before the horse. The goal is, presumably after all, to live happily.
I would not expect a man in his late 20s to fall into the category "men of an advanced age". But I would be unsurprised to learn that there was visible protocancer in a sample of his prostate.