First off, you say "besides age" as if age isn't a big deal, particularly at this moment in time for the Democratic Party. It is a huge deal for voters, particularly after Biden was forced to drop out in 2024, after RBG hung on too long to her seat, after watching four D representatives keel over in the past year and a half in a very divided house.
But even setting aside the age thing, Spanberger and Sherrill both ran races that were effective to winning support over to their side in the primaries and both were clearly up the challenge of the jobs they were running. Mills, with the campaign she ran, didn't demonstrate any of these things.
Voters are sending a message here, whether you and I like it or not. Hope the party establishment learns going forward.... to be fair, they have had some recruiting successes this time around landing Cooper, Peltola, getting Brown to run again, etc., but this was a pretty clear miss on their part.