I understand your complaint in the larger context of popular culture. But in the context of Dune, which takes place 1000s of years in the future, and in which there exist a genetically manipulating pseudo-cult group of witches/spies who are desperately seeking the ascendancy of humankind to a higher plane of existence, it is slightly more reasonable that Jessica Atreides looks the way she does, as a hyper attractive concubine that was intended (you might say "designed") to eventually develop the bloodline of a pseudo-god. But that also only really makes sense if you're invested in the narrative.

The real kicker is that Paul probably should have looked younger. In the novel, he arrives on Arrakis when he is 15.