In which chapter is that stated ?
Also, another reason why I can hardly accept Guts is anywhere below 25 is that Miura hardly mentions anywhere how fast is the story actually preceeding (how many days/weeks/months do the adventures and the traveling between places takes), the only two occasions I remember is where it pointed out how many years he spent with the Hawks and how long he was out on a lonely revenge quest before returning to Godo's mine (and finding out Caska went missing). So I think it is quite rightful to assume he's around 30 by now.