My wife and I are a few years older than you and we have the same problem! 20 years ago there were a number of venues offering dinner, live entertainment, music and dancing and attracting people of all ages. These have all disappeared.
As you rightly point out, the focus is on "bars" although what Icelanders call bars and what you call bars may be a bit different - here we're talking about somewhere serving drinks and with live music or a DJ and which is really active from something after midnight until 05:00 during the day these "bars" are coffee houses / bars serving what is often good and reasonably priced (by Icelandic standards!) food.
The best suggestion I can offer is that these same places often have a mixed age clientele in the evenings (say, until around 22:00). Other than that, some of the more up-market restaurants, particularly those in hotels, have bar lounges, but no music.