Yeah, there have been several studies that show that the vast majority of people stop discovering new music around the age of 30. Some say 33.
The most likely reason, is that from about one's mid teens up through their mid to late 20's, people are experiencing some of the best times of their lives. And the music they and their peer group listen to, becomes the "soundtrack of the best times of their lives". New loves, great wild parties with friends, going away to school, newly discovered independence, etc, etc, all with the music of their time playing while it is all going on.
Their experiences become inseparable from the music.
Music that comes after that time period of their lives, even if it is qualitatively no worse (or no better), does not have the same kind of connection. It is 'only music' now, it is not their soundtrack.
I actually have hardly any nostalgic feelings connected to music at all. I am never trying to relive my youth through music. For me, music lives or dies based on its own musical merits. Either it has the attributes I love in music, or it doesn't.