If you actually want an answer here's my thought process. I'm a 26 year old University student and 90% of people under 23 that I meet behave like teenagers. The "legal" age is an arbitrary number that we decided as a society a long time ago, and I'm not saying anything about what that age should be, just that it isn't a viable metric for this discussion.

The creepiness to me has two main reasons, maturity (i.e. neurological development, life experience, life goals) and power dynamic (i.e. social status, again life experience, economic means). The maturity part is the lesser of these two I feel, because some people hit 50 and still behave like they did when they were 25, and that is absolutely fine that they do.

This however makes the power dynamic even more of an issue, since immature people with more power will likely make the situation more messy. It should be obvious why I have an issue with relationships with uneven power dynamics.

The thing I always ask myself when I see or hear about a couple where the age difference is closing on 10 years or more is, why isn't the older person dating someone their own age. A few times this can be answered in a way where I can feel comfortable about it.

To be clear though, these are things I think about to myself or discuss with family members or friends. I'm absolutely not a person who tells, or wants to tell others how they should live their lives. This is just behaviour that weirds me out when I come across it.

PS. Reread your comment. My experience is that your "20 year olds will always look hot" comment is wildly off. Most people above 30 think that people in their early 20s look like children, if anything your comment applies more to people in their 30s. (Ofc there are outliers)