While many of the proposed explanations (anchoring, lack of discretionary time, etc) may be valid, I think the biggest factor is probably the social network: we

While many of the proposed explanations (anchoring, lack of discretionary time, etc) may be valid, I think the biggest factor is probably the social network: we are exposed to new music when we spend time with new people in a setting where they're selecting the music. And those kinds of social situations generally decline with age. You spend less time with new people, and less time in situations where there's a lot of music (clubs, house parties, riding around with friends in the car, etc).