Dragon Ball Z is not in the public domain. Goku’s age is unknown, as there has been no definitive date of his birth released yet.
Even if we can be sure that Goku first flew to outer space when he was maybe six (or eight), it remains unknown how long ago that was, too. So for now, while we do have a rough idea of his approximate age according to the manga, and based on what power levels he’s supposed to have achieved at different points in the series – but until an official story comes out with a time frame of some kind – we’re stuck waiting. Again!