While it may not necessarily be a bad thing as gore isn't everything, I don't see the point in making a toned down version. Alien and Predator was meant for adults and it should stay that way, I mean look at AVP, out of its many faults one of them (to some) was that it was pg13, Alien and Predator come from an R rated background and a lot of the kills are horrific, taking them away from their elements doesn't really work.
Also, why specifically aim it at young adults? The only thing that springs to mind is that its going to contain some cheesy horrible romance arc.