For HGH to work on height it should always be taken before the bones fuse as the following illutration clearly proves:
The tallest man is Leonid Stadnik of Ukraine; he is 8.37 feet or 2 meters 55 centimeters tall and is still growing. This is ironic since most people stop gaining height by 21 at most; 16 years later and this guy is still growing which means that his bones never fused. The question is why his bones did not fuse yet? maybe something in his body did not allow this to happen. According to Leonid who sees his height as more of an annoyance since it is making his live very complicated, the reason he kept growing was due to a surgery that he had on his brain when he was 13. This surgery in turn caused his brain to overproduce HGH without letting up after puberty which generally should happen to allow the bones to fuse. If too much HGH is always present before the fusing takes place then the bones continue to grow. In a way Leonid is lucky that this happened during puberty; if it happened after puberty once all bones have fused the massive amounts of hgh in the blood would cause every organ, cartillage to grow but without the verticl bone growth to create new space for the new organ growth. This in turn would have caused all kinds of problems limiting further his movement and making his quality of life very poor.