The city of Reykjavík performed a study on voting by age for the first time in 2014 for Reykjavík's two constituencies. So breakdown of the election turnout is a relatively new addition to elections in Iceland.
The pending numbers for the most recent election have not been published yet, but the city has published an excellent analysis on the 2014 municipality elections, which had the lowest turnout since 1928.
The Reykjavík turnout data is plotted here as a population pyramid of the registered voters, with the turnout highlighted. It's very obvious, as it was in the analysis, that the "younger generation" doesn't show up to vote. Why? We don't know. Has it maybe always been this way? We also don't know. This was the first time the breakdown was available.
Another curiosity is the lower turnout amongst females over 70 years old, compared to males over 70. Women have had full voting rights in Iceland since 1915, and even longer in municipality elections, so there are very few women alive in Iceland today that haven't always had the right to vote. We can guess that this might still be a sign of times past but since we don't have data, again, we just don't know why.