Musically? About 9, when I started playing clarinet. I got to hear what instruments really sounded like. My aunt and uncle started taking me to classical concerts then, as well. Moreover, around the same time, I met a good friend whose father was a hard-core audiophile. We weren't allowed to touch the gear (and we didn't), but were welcome to stare and he was happy to play it if we asked.
As for gear, I got a pair of MDR-V6 in undergrad... I was probably around 20. Got pulled into tubes, antique radios and amateur radio around 25. That led to building DIY ribbon loudspeakers at 28. Those ribbons really opened me up to the hobby. The bass response falls off around 350Hz, but they're flat and creepily real above that. And the dipole radiation freaks out first time listeners.
Anyhow, I got a catalog with the HD-650 in it around December 2005. So I bought them and a Creek OBH-11. That sounded great, so I searched around to see if anyone else liked headphones....