Prostate cancer can rarely occur before age 40 years old, but its incidence is affected largely by age. The incidence among age 40 to 44 would be about 7 in 100,000. For the age group of 70 to 74 it is 900 in 100,000.
but still need to be careful, younger patients can present with PIN, a pre-cancerous condition of the prostate. so it's wise to do digital exam, PSA and TRUS+/- biopsy.