Do older (20+ years old) people tend to cheat? ?

Everyone 20 and older cheat.

Only people who are not a good partner 

20+ might be older than a child but as adults go it's still right in the pocket of the peak of male sexual interest. Guys' testosterone levels don't start to wane until closer to 30. So if he's cheated in the past chances are he may do so again. However, just because a guy has a monster libido doesn't mean he lacks character. Plenty of young men can stay monogamous if they want to. 

From the statistics I've seen and that you can search, the older people become, the more likely they are to cheat.

On a 2016 General Social survey, 11% of 18-34 year olds, 17% of 35-64 year olds and 18% of 65+ year olds reported to having cheated.

I doubt someone would cheat in a new relationship, but the longer it lasts, the more likely it becomes that they lose interest.