Thanks to many factors - from online dating sites, to modern medicine, new and improved vitamins, and even because of a poor economy - more senior citizens today have active sex lives. That means they are also more likely to get sexually transmitted diseases.
In the past 5 years, the rate of STDs among active seniors has risen over 70%. The rates of STIs among older adults, including syphilis and gonorrhea, have doubled in the past decade, and HIV infections have almost doubled as well.
So, if you've got to do the nasty with grandma or grandpa, then make sure to do it safely.
Source: Safer Sex for Seniors