Since it's behind a pay wall, here's a short summary: A study was done using data from the Ashley-Madison dating web site. For those who missed last year's headlines, this site is for people already in a committed relationship who want to find someone to cheat with. Researchers looked at the ages of registered users and found a statistically significant bump in the distribution for ages ending in "9".
The conclusion is that people are more likely to cheat when they are approaching a milestone birthday (30, 40, 50, etc.). They get stressed about aging and reach out for a little excitement.
That's certainly a possible explanation. However, I think they overlook another simpler explanation - people lie about their age. If you're anywhere in the 40 to 45 range and you want to claim you're in your 30's then 39 is more believable than 38, 37, 36, etc.
I think this explanation is at least as likely as the first. Keep in mind, everyone on this web site is trying to CHEAT. They're all dishonest. Why wouldn't they lie about their age?