User blog comment:Kallutofan990/Why does Killua's age say his 14 currently?/@comment-4828844-20131107143307

During the beginning od the series, he is really twelve years old (same as Gon). But time passed too. From Heavens Arena to Yorknew City arc they spent 6 months or so. I don't know how long they stayed in Greed Island and in NGL. If you compare Killua during the Hunter Exam and the Killua who is caring for Alluka now, you can see how much he has matured (also physically, I guess).

The fourteen year old age is probably just a speculation. If it is stated in the manga, then someone should provide references.