I was talking old cameras with Simon and so looked into the Lightroom to see what the oldest photo was. 10 Feb 2002. Taken with a FujiPix2002 (2.1 mexapixels). The filename is DSCF0002 so presumably the first was lost to time.
I had taken many photos with film cameras but this is - as far as I can tell - the earliest digital photo I have.