I'm a research psychologist and have studied this issue. If you'll forgive a shameless plug for my book, The Things We Love, I've got a discussion of the research on this issue in chapter 7.
As some people here have noted, not everyone does stop liking new music as they get older. But it is very common. There are three main reasons:
1. Neurological changes in the brain as it matures.
2. Our social situations change. When we're young, we're in a lot of social situations where we are exposed to new music, e.g., listening to music with friends. As we get older, that happens less, and we only get exposed to new music if we make an effort for it to happen.
3. When people are young, music is a main way we define our identity, so it is important to us and we talk and think about it a lot. As we get older, our identity shifts from what music we like to our career, our family, our home, our choice of vacations, our possessions, and lots of other stuff. So for many people, music becomes a smaller part of their lives.
The book is about why people love things, including music and audio equipment. It might be of interest to people on this list. And Amazon named it a "best book" of 2022. So I hope you'll check it out.