Danmachi (Is it wrong to try to pick up girls in a dungeon - blame the editor for the title, the author originally wanted one matching the story more closely) comes to mind, that series has been going on for over a decade now and, at least for me, it has been getting better and better with every volume. It's got its problems (mostly certain characters being overly annoying) but it's awesome nonetheless. Even now (I've read up to and including Volume 17) there are still questions open from the first Volume with most plot developments only deepening existing plot threads instead of opening up new ones.

Disclaimer: Danmachi is also the story that made me start reading Light Novels so my take on it is bound to be extremely biased.

I pick Spice and Wolf. I think that was only my second light novel Light Novel I've ever read. I started reading the fan translation online after the anime was announced and I've heard people talk positively about the series. So that must be around 2007. And to this day there weren't many series that had that much depth.

I have had a look through my read series and it looks like only Death March to another world rhapsody and spice and Wolf (listened to the first few audiobooks) are older than 10 years. Unless you include the original Novels then a few others like In the land of Leadale would be included (but I never read them).

Death March is one of my top series so it is defiantly my favourite over 10 years old. I just wish they would release it quicker.