It's the same as it used to be, it's us who change.
Every generation tends to think the music (and general culture) of their youth was the best, and especially compared to that of the following ones. I think a lot of that is to do with selective memory. You only recall the best of the kind of music you enjoyed when you were younger and not all the mediocre and rubbish that was around too. If you're listening to general pop music today, you're getting the good and bad together.
Fashion plays a major role too. Loads of people will subconsciously follow the crowd (even the hipsters who like to think they don't) and the music industry have long recognised that so put out a lot of the same style of music to try to get on the current bandwagons. Again, we only remember the best examples from the past and not all of the second-rate copies, but hear all of it with current music.
Well you're not 16-25 any more so why would you expect it to still appeal to you? Your life will be different, your emotions will be different, your brain will be different. I'm sure that if you could hear the music of your youth, but without the associated memories and emotions, you wouldn't think much of it today.
It's also worth noting that you didn't say how old you now are or which generation of music you're comparing to, but I'm confident my point will apply regardless.