Two paths off the top of my head... 1. Time Travel. In the series Vamps do age but through corruption of the human form. (think the Master who couldn't go back to human or Kakistos who's hands and feet were cloven). So have a 500 year old Spike come back in time. 2 Pushing aside the comics and have Spike get the Shanshu Prize after Wolfram Hart fell in Series Finale of Angel. (In stead of Angel turning Human) So he just aged like a normal person.