The later that T levels fall below 12.1 nmol/L, the lower the risk. The article says:
In a study of 376 untreated hypogonadal men aged 45 to 74 years, Prostate Cancer developed in 26 men during 2 to 11 years of follow-up, according to findings published in the Journal of Urology. The older a man was when his total testosterone fell to 12.1 nmol/L or below, the lower his lifetime risk of Prostate Cancer. The risk declined by 32% for each 1-year increase in age at diagnosis.