The chances of a male developing prostate cancer are roughly equal to your age.
Without risk factors, an annual PSA and physical exam (finger up the ass--an abnormal prostate texture can indicate cancer) beginning at about 50 should be sufficient. With risk factors, such as close blood relatives who've had it, let your physician, or better yet, a urologist, tell you how to handle monitoring but you should probably start at 40.
There are many types of prostate cancers. Most men get ones that are fairly slow growing. Indeed, a good number of men die with prostate cancer without ever knowing they had it because it was asymptomatic and something else took them first. Other men end up dead within weeks or months of the cancer being found because it was an aggressive form (this can happen at any age). And everything in between happens. If you are a fan of the Mitch Rapp thriller novels, the author of the original fourteen of them died of a rare aggressive form of prostate cancer at 47.
I skipped an annual physical once for all kinds of the usual reasons and ended up at my next physical with a PSA in the 20s. I had a retest the next week, a biopsy within about a month (I don't remember it being very painful, just like being poked a few times), and I had a prostatectomy less than three months later plus several months of radiation treatment after that. Currently have a PSA check every three to six months, dependent on the last reading, and a PET scan about every other year. which will probably be my program for the rest of my life. My PSA is very low, but not quite as low as is ideal post-surgery, and since the surgery has ranged from very slowly increasing to stable to even slightly declining. The PET scans consistently show no cancer. The current working theory is that there was some healthy prostate tissue left after the surgery that is producing the PSA. (They can't get every bit of prostate tissue out without running the risk of bladder or urethra damage.)