I'm a 38 year old woman, and I literally cannot see men in that age group as potential sexual partners. They're like invisible to me because they look like kids. On the rare ocassion I do notice a good-looking guy that age, I think how if I met a guy like that when I was his age, I'd go for him. Interestingly, in my early 30s, I was able to be attracted to 20-something men.
Recently, I worked closely with early 20-something interns, and while they were very professional and mature in the work setting, their personal conversations between themselves were so young and immature. They sounded exactly like teenagers. Even if they are mature for their age, the sheer lack of life experience thwarts any kind of meaningful connection. I never used to have a strong opinion about older men/younger women preferences, but after that experience, I find myself being more judgemental. If you're a man my age and older who pursues people of that maturity level and/or inexperience, something is off.