He doesnt really have an age. The original image still hasnt been tracked down and the 2011 story where he is 13 isnt technically his actual story, the guy who gave him the name jeff the killer actually made him like 30 and Liu is a single father lol. Its a mess
edit: as im further into the game, he mentions dropping acid on himself in reference to the scars, which is in line with the ''original'' lore (its very complicated and dumb, but basically hes like 30 here)