I am an older guy and this pua stuff was not public knowlege when I was younger but I wish it had been. I had to learn some painfull lessons, walk up hill both ways, etc.
Here is an old quote that is totally and completely true "Youth is wasted on the young". What that means is that when you get older, you look back and you see all of the shit you could have done if you had not been in such a hurry to be a grown up. You also see all misconceptions and wishfull thinking you had.
Cheating and Age: Cheating is more about the psychology of the people involved and the amount of opportunity to cheat. Young good looking girls have unlimited opportunity and are not psychologically prepared for a real relationship. Many of them will enter a relationship but it it is because they are supposed to or because they are insecure or they want a baby etc......not because they are really good relationship material.
As a general rule, do not expect any relationship to last more than a couple of years, or be completely monogamous, if either person is under 25. This is even if you have a child or get married. In the old days culture and religion helped hold these relationships together, but that doesn't mean they were happy or that no cheating went on. Bottom line, under 25, work on yourself and get experiences. I wish I had.
Having said that, older people cheat also, but if two psychologically healthy people are compatible and have a common desire to stick together they probably will. If the couple is older there is a better chance that they both know what they want and have held out for a good match and found one. Those are the happy couples you read about that have been together for 30 years. They are rare, but the do exist. There is a very high divorce rate, but in my opinion most people who get divorced should not have ever gotten married.