yeah i know about the whole bra with goten and pan with trunks, but cud they have any other parents that could make them as powerful? if u recall pan is gotens neice, not sister, and btw this takes place 25 years after gt so it wudnt matter about the age difference, plus saiyans grow up a lot slower than humans, and can live a lot longer than humans.
If you watch the end of GT u'll know where goku is, he was abosorbed and became 1 with the earth dragon.