The older you get, the weaker your immune system. Also, as people age it is more likely that they will have microscopic lesions at the slightest friction. Thus, old people are more likely be infected with a STD at genital contacts and have worse symptoms and disease outcomes. There are some STD's without any treatment, mostly viral infection including HIV. Remember the Tom Hanks character in the film "Philadelphia" covered with weird rashes? That is what happens when someone with a compromised immune system gets herpes. That is whats is going to happen to old people if they get herpes.