Why Do Many Humans Become Less Interested in New Music as They Grow Older?

Because as humans grow older, their interests, focus, and priorities in life change.

When a person is in his teens especially, the sense of discovery and craving for something new is normally at its peak (After all, in human development, childhood and youth are the stages when an individual is most impressionable and excited for new things to experience); so, most people, when asked what kind of music they think are the best, claim that the music during their youthful days are the best; whereas the current, lackluster; but this is not true. They simply could no longer feel moved by new music because their sense of discovery had already waned and their budget for hobbies (like in school days) is now better allotted on family needs (They have become the providers of necessities and the payers of bills, tuition fees, kids' school allowances, etc.), so they have also become out of touch and could no longer catch up. 

Yes, the claim that there is no more good music these days is incorrect. There will always be good music, from any given generation since the beginning of recorded music's commercial availability. It's just that the older people become less interested, jaded, and have lesser money to spend on it.

Even many musicians I know have gotten stuck with the music of their heyday; they find liking new stuff difficult now.

Personally, I chose to become a music journalist, reviewer, and collector--this is primarily the reason I remain in touch with new releases. Add to that, being an album reviewer, I get to receive lists of upcoming releases and advanced copies of those that I want to review, from my publisher and from record labels; more so, being in Canada, where there are still a good number of record shops, I could buy music regularly. Most importantly, I have long developed a strong passion for music of various genres, so I always crave for it. If not for these, then I might have been among those who have long become outdated and jaded.

Lastly, in the current generation, there are already a lot of accessible forms of entertainment--no longer only a few that include music. So, younger kids have lots of choices. Music is no longer a top priority. Even if it is, there are already lots of platforms where one could listen to it anywhere, anytime. 

We cannot stop cultural evolution. We could only try to, at least, hold on to some of it and try to the best we can to influence our friends to stay excited or inspired in learning new items of culture such as books, films, and music.