...So if its for 30 to 40 year old's to bring there kids, wouldn't that  mean the main demographic is the 30 and 40's? cause other wise, their  kids cant see it, or better yet, what about the middle aged people  without kids? Are you saying they for the most part wont have an  interest in seeing it? 
Also, I'm 22 (so are most of my friends) We  all went to see this movie for the nostalgia of seeing the heroes of our  childhood kicking ass again. I believe the demographic is more like  20-50 year old males. Cause not only do the people in this age group  KNOW who the actors in the movie are, they're also old enough to go see  an R-rated movie. 
Also, CGI blood is used (Not just this movie,  but most movies) because using digital effects means its easier to reset  the sets to do another take. You know, instead of spending 1 hour  cleaning up the blood, 3 hours making sure the blood is consistent  between takes, ect.