I saw this in the theater with kids ages 7, 6, 4 and 3. I loved it. It actually had me laughing out loud in parts. Lots of older kid and very tame adult humor. Here's what we thought:
6 and 7 year olds: Loved it! (They both like the Ice Ace movies and said The Croods is better.) They did think the chase scenes were scary but those scenes are over quickly and aren't gruesome. They questioned why the family had to leave their home and what all the lava was about (i.e. end of the world conversation).
3 and 4 year olds: The younger ones enjoyed it and sat in their seats the whole time (a first!), but it was a bit scary for them. They missed a lot of the inferences to death, safety, and the end of the world, but there are several chase scenes with wild scary animals, and while the people are meant to resemble a humorous depiction of cavemen, they were a little too wild and crazy for this age. My 3 year old had nightmares the night after seeing the movie, telling me he was afraid the mean tiger was going to eat him.
Overall: This movie moves fast and is super entertaining. No gross blood and guts, and there aren't any scary death scenes with the exception of a creepy scene where killer birds flock together and eat an entire animal, leaving the bones behind. Be prepared to discuss the end of the world concept. There are some violent type actions between the Crood family members - bonking heads, running over each other, etc. - reminiscent of Looney Tunes. This movie is definitely NOT for kids under 5 or 6, and the older they are the more they'll enjoy it.