I think collage is a wonderful way to get kids interested in art. It is doing wonders for my 4yo who doesn't want to draw or write because, he can't do it perfectly. I have noticed his coloring sheets from church are coming home with more and more crayon on them since he has started doing collages.
Plus, it isn't an age thing. I can happily do collages myself. Just change the materials.
We like to use:
foam shapes, foam sheets, foam letters and numbers (these can be hilarious with words on the collage, a recent one had T-O-E on it, ha ha)
pom poms
googley eyes
torn paper
construction paper
markers and crayons
I also like to make little flags with paper and popsicle sticks. That is something I did as a kid and have started doing with my DS. I used to just make them up, but DS is obsessed with geography so we have been doing real ones.