My older digital cameras have all perished; that makes this question very easy to answer: the Leica M8 is my oldest fully working digital camera - and it's still going strong (one of my favourite day-to-day carry cameras). Mine was made in 2008, but I acquired it in 2018. The Panasonic GF1 is almost the same age.
I graduated from a Nikon P50 to a D90 in 2009; before that, I had experimented with digital, but not taken it seriously. I had owned a Nikon 600 (the first "compact" digital Nikon - 1MP!) for quite a while before buying the P50 - and a Kyocera S3 which was a truely fiddly camera to use (but took way better images than the 600). The P50 is annoyingly close to being a useful little camera - and I still have it somewhere. However, after the acquistion of the D90, I never looked back. But not even that camera sold me on digital - that was the Olympus E-PM1's job, 2012 ...
M.