While turning my CD collection into MP3s (which is taking a lot longer than I thought), I determined that I grew up sometime in 1995, the year I turned 26. That was also the year I got married and quit my band -- though I've since re-joined the band now that we travel less and make way more money.
But my CD collection told me for sure because 1995 was the last year when I bought a lot of music from new artists. I still do that, but only occasionally. In the early '90s I grabbed albums from Juliana Hatfield, Midnight Oil, Nirvana, Joan Osborne, Rage Against the Machine, Crowded House, Green Day, Junior Brown, Cracker, the Tragically Hip, and other artists who were then in the bloom of their careers. Now, I'm still buying albums from many of those same artists and their descendants (plus older music, like that of the Beatles and the Nuggets '60s collection), and hardly any from anyone who's emerged in the last two or three or five years.
I don't own a single Radiohead CD, for instance, even though they were around and had some minor hits a decade ago, because they broke big after 1995. Had ok computer or Kid A happened in 1994, I'd own them.
In retrospect, I should have realized what was happening in late 1995, when I read a magazine article that asked various stars what their favourite summer music was. Gwyneth Paltrow replied that she liked Tom Petty's Full Moon Fever album, from 1988, the year I entered my third year of university. In the article, Paltrow mentioned that she'd been a junior in high school when it came out, and I recall thinking, "Jeez, she's just a kid!"