The oldest solved cold case was widely believed to have been the murder of Maria Ridulph, but it turned out the man they eventually found and convicted in 2012 for her murder was exonerated, released, and officially declared innocent in 2017.
In about ten minutes of Googling I’ve been able to find this article about the murder of Wendy Jo Halison, solved after 48 years.
Then I found this one about the murder of Loren Sundholm, solved after 51 years, but the suspect died in 2009.
Is there an older one?