It’s definitely true that the music we love in our youth sticks with us and becomes part of us in a way that great music we hear later in life doesn’t.
But I find that the songs my son loves or has loved are becoming part of my soul, and part of the soundtrack to my life… even when the songs themselves aren’t all that great.
I understand the question behind the OP - there are a few years during which the music is incredibly important to me.
On the other hand, I am over 50 and I am still similarly affected by songs like this one:
Often, one loves a song not because it is such a great song, but for the moment that it evokes. Some of my soul songs are do-wop – how good can they be?
I even have a Demis Roussos song in my soul list, for my days in Khartoum, a city I loved.
Something special happens when we hear an old favorite for the first time in years. A lot of memories from that time in our life come flooding back.
That happened to me for awhile after I discovered YouTube. I found several old songs from my high school years that aren’t routinely played on most oldies stations. I got pretty sentimental at the time.
I doubt I’ll have that strong reaction to John Legend or Adele. I like their music. But, hearing one of their hits 15 years from now probably won’t be the same. I won’t get sentimental about the 2000’s.
I’m like this as well…or maybe I should say that I had two waves of it. The first was the British Invasion stuff which really I’m too young for, and the second was grunge which I’m almost too old for (I just turned 50). The Beatles in particular had a huge influence on me as a kid, and then the wave of alternative music produced by people my age in the early 90s was the stuff that I really personally identified with. I doubt I will ever have any music impact me or become part of me in the same way ever again.
To use a specific example… I don’t really like “Dynamite” by Taio Cruz, but that song will always spark memories of my little son dancing around the living room. So, it will NEVER be a favorite song of mine, but it kind of IS part of my soul. It’s part of the soundtrack of my son’s childhood, so it’s part of mine.
I’m too old to love rap in general or Eminem in particular, but Eminem is part of the background for all kinds of experiences I’ve shared with my son. HIS songs are BOUND to be my songs, in some way.
PS Alas, my Mom will never regard my Seventies metal albums that way. Zeppelin will never be part of her soul.
Hey cool, someone started a thread based on an offhand remark I made in a different thread. Can’t recall the last time that’s happened. 
I think what most everyone has said so far is correct, especially how youth plays a major factor. Of course, it’s not an absolute rule – I started listening to Porcupine Tree in my early 40’s and by now it’s certainly become soul-based. There’s also the fact that there’s only so many types of music out there, and once you’re exposed to them all, discovering a band in a pre-existing music genre isn’t going to have the impact it would have otherwise. Getting into Metallica at age 14 is a much more profound experience than getting into Devildriver at 35.
Regarding LedZep, it’s something I’ve notice about all my friends who’ve listened to Zeppelin – all of them started listening to them between the ages of 16 and 25, but after that, they basically moved on to other things. That doesn’t mean they don’t like them anymore, far from it. It’s just that LedZep’s music isn’t as “mature” as other forms of music, for want of a better word.
There’s also the timing element, being in the scene when it actually happens. Someone who gets into Nirvana today will never be affected as deeply as someone who got into them in the early 90’s. Same could be said for The Doors in the late 60’s, or the Sex Pistols in the late 70’s. Again, that’s not a hard-and-fast rule, as there’s millions of Beatles fans who weren’t even alive when they were active.
By the way, I sort of cribbed my line from the opera scene in Pretty Woman – so sue me.