This is looking like an absolutely awful result for the SDP even if they top the poll, pretty comparable to the "win" of Swedish S in 2014. Their second-worst result ever. If their highs look like this, I don't think they want to know what their lows may look like.
The SDP is very much a party of smaller towns; always was. Given longterm population movements this is a little bit problematic and means that the party has undergone a pretty substantial structural retrenchment in the scope and scale of its support. This is actually a bigger problem than the fact that its electorate is older than average; that, in itself, is not automatically a problem.