In one study the odds or remaining potent fell with age (Cancer 1997;80:442) > 90% for age < 60y,  80% if 60 -70y and only 50% if older than 70. In another study Zelefsky (IJROBP 2000;47:1261) reported: Two hundred twenty-one patients (89%) were potent or able to achieve an erection sufficient for intercourse prior to TPI. Sixty-four patients (29%) developed impotence after TPI. The 2- and 5-year likelihood of post-treatment erectile dysfunction among patients who were initially potent before therapy were 21% and 43%, respectively. So the risk of impotence goes up with time and may be higher than initially reported after seed implants.

There is conflicting data about impotence and the radiation dose to the penile bulb (go here).