This is from Eton's website: "A boy may be registered up until 30 June of the school year he turns 10 (UK school Year 5). After this the only route of entry will be through scholarships or Sixth Form entry, which open in Year 8 (for Year 9 entry) and Year 11 (for Sixth Form entry)."

It sounds like George is taking the entrance exam so he can be registered at Eton during Year Five to start when he's 13.