Record of the Lodoss War (a D&D campaign turned into anime, if I recall rightly)

Mixed-age cast—the main characters are adults in the context of their society, but not all are necessarily 20+ years of age. Also be aware that there are two different versions of this, an OAV and a later TV series. There are significant differences in story between the two (the TV series adapts more of the original material, the OAV ends differently). I prefer the OAV, but it's mostly a matter of taste.

.hack//sign (dot hack sign) is a pretty interesting one. It has several other series in its ‘canon’ if you like it. I’d say it was the adult oriented, well-presented premise of sword art online before sword art online was wet-dreamed up. A friend described it as ‘waving its dick around because they had an actual orchestra for the soundtrack’ in several scenes.

The last time I watched it (admittedly about ten years ago now), I found that it hadn't aged all that well. The loose ends left because it was a prequel to some of the games in the .hack franchise were very noticeable. (It's still a better version of the "trapped in a game" trope than SAO, though, because it doesn't repeatedly reduce the female lead to a damsel in distress). And not all the main characters were adult, although some clearly were. The soundtrack is still excellent if you like Yuki Kajiura's style, though.