I don't believe her actual age is given in either series and since she is a modified homunculus, she ages much slower than an average person.
Based on evidence given in the Fate series, approximately how old is Illyasviel von Einzbern in Fate/Stay Night and Fate/Zero?
At the time of Fate/Zero, Illya is 8 years old. In Fate/Stay Night, which takes place 10 years after Fate/Zero, she is 18 years old.
It was mentioned in Fate/Zero, that the preparations for the 4th Holy Grail War started 8 years before, where you can see that Illya is born. The 5th war happens 10 years after the events of Fate/Zero making her 18 years old.
