I think that with a little coaching and "helping" a 5-yr old should be able to have fun with it. I agree that a no-time limit for them would be a good thing. The fact that the game limits where the models can go and how many things they can do each turn is definitely helpful in letting younger children participate.
Unfortunately, the image that pops into my head now is Joe Camel, I've just realized that this is how GW gets the kids at a younger age...
MZ