From my experience in playing Sims 2, the want trees relating to plantsims are very different from those for other creatures. If a sim becomes a witch, for example, other non-knowledge sims will usually only roll fears relating to becoming a witch, while non-knowledge friends or lovers of newly-turned witches will want them cured. With Plantsims, it's a different story. If a sim becomes a plantsim, any non-knowledge sim who knows him or her will roll wants for that sim to be cured. Even with wfsanity from More Awesome Than You, non-knowledge plantsims will still roll fears of their lovers becoming plantsims. I'm not so sure about non-knowledge witches fearing others becoming witches with that mod in place.
Sims with Knowledge as a secondary aspiration will still fear becoming plantsims, while wanting to become witches and werewolves. They tend to get iffy once they turn, and when they turn others into werewolves, but seem to be fine with turning others into witches.
Young adults, despite officially not being able to become plantsims(though a glitch may allow them to do so), they will still fear either want or fear turning into one.
Teens, however, will not have any witch-related wants, regardless of aspiration. The witch want trees only affect adults and elders, much like the plantsim want trees. Teens will still have wants and fears related to vampires, werewolves and possibly zombies. I don't know for sure yet, but when a sim dies, I will likely find out. They have wants and fears relating to Bigfoot as well, and will either want or fear meeting him, while children will want to see him. I have modified the 'see wolf' wants in my game so that children will want to see one(kind of makes sense, because kids often do like to see scary animals at the zoo).
Studying want trees can be quite informative, and if you know what you're doing(and back them up first), you can customize them in case you want to make a paranormal-themed custom hood without sending most of the population into aspiration failure.