Sleeping with older partners isn't safer..
They might have, on average, a better record of having safe sex but one of the growth areas for STIs is in fact : 45+. Newly divorced, shagging around and thinking it's safe because they've been made infertile. At that age, or older, practically everyone has herpes (oh, sorry, it's not herpes when it's a 'cold sore' you got once on your mouth, is it?).
There are, sadly, also sometimes specific communities where sleeping with older people has a significantly higher risk of contracting various STIs.
Given that people, for the most part, stick to partners within a reasonably narrowly defined age range it's not that unreasonable to suggest that straying outside that range exposes you to a different risk profile. That does, however equate to catching different STIs - not an increased risk.
To say that a greater than five year difference automatically provides a greater risk is obviously bollocks, although I do like one of the commenter's theories that the people who routinely have sex outside that age range tend to follow a higher risk lifestyle.