Isn't that an issue then? Shirou is still a high schooler, Rin is still basically the same age too, but went to the Clock Tower early.
If those things are still the same, doesn't that mean that Illya is still older than the both of them?
(Illya is 8 at the beginning of the Fate/Zero Grail War, 10 years pass before Shirou/Rin are high school age.)