The one I read said Gon was 12, and that's when he went on the date with Palm. Gon started out at age 11 in the Hunter Exam, and his birthday's in May. So sometime during Heaven's Arena he turned 12, and he's been 12 through Greed Island and the Ant Arc. He would have turned 13 sometime during the end of the Ant Arc, as the election arc took place approximately a year after Heaven's Arena in HxH time. Since everyone had their birthdays around the same time, it can be assumed that our main four are 2 years older than they were when they met during the exam. Gon's 13, Killua's 14 (or 13 depending on how you interpret the manga), Leorio's 21, and Kurapika's 19.
As far as their ages go I have a question. It was said in one of the translations during the chimera ant arc that Gon was still 11? I'm not sure if it's true it wouldn't make sense since I see him being at least 12 by now anyone have any definite answer on Leorio Kuripika Gon and Killua's ages according to the current story line?