I already posted about how I was in on Blink 182 before you were, even if you don't care. If you do care, I probably ditched them before you did. I was done with them before their second album album came out, so I didn't get into anything off Dude Ranch until Pete and I started ironically listening to their Greatest Hits while playing Super Mario 3. I grew to love this one and "Josie" as my favorite songs by them, period. I almost chose "Josie" instead because of the title but upon serious consideration, this one wins.
I don't feel like I "grew up" per se until I left grad school after my mom passed and started my first full-time job, at which time I was already in my late 20s. It was around that time where I felt like I was less in contact with my best ladypals from high school (with whom I jammed out to the first Blink album, attended the only concert at which I ever saw them). Pete and I were going through some personal stuff. I was in the midst of a big post-parent-loss depression I didn't even realize was happening and definitely drinking too much. I had unchecked thyroid issues, gained a bunch of weight. I dunno, it was a weird time.
I recall distinctly talking to my best friend on the phone in a cab on the way back from a business trip about what she was going to do with her life next. I thought about this song and wanted to sob so hard. Which is so dumb and embarrassing because "I guess this is growing up" was probably a throwaway anthemy/breakthrough/intentional sellout hit chorus line to Blink but to paraphrase Arrested Development, at the time, it meant a great deal to me.