A cougar is a woman who pursues younger men, since cougars are predators, so it doesn't just indicate the age difference between the woman and the man, but her behavior as the alpha in the dating game. If you are a younger man pursuing an older woman, then that makes that woman "prey" and not a cougar, regardless of how old she is. At least, this is the argument I use whenever anyone accuses me of being a cougar because of TheBoyfriend. He chased me, there's no question about that! I just let him catch me, eventually.
Since being called a cougar several times because of TheBoyfriend, and I was extremely put-off by it the first three times I was called one, I've asked every single one of my friends what they thought of it and whether they thought it applied to me. Every single woman thought that the term "cougar" was extremely offensive and they were very insulted that I had been called one because they felt I wasn't nearly old enough. Women clearly hear an insult in the term, and especially "old" implied in it. My male friends thought that the term was either really funny, laughing hysterically when they found out I'd been called one, and/or very flattering. One male friend consoled me by telling me that "No one ever calls an unattractive woman a cougar" which did make me feel better. All of my male friends over the age of 30 also said, "You aren't old enough!" but my male friends in their 20s thought the term was apropos. This has led me to conclude that the generation behind mine must define the term differently than my generation does.