The rule in our house is that as soon as you can walk you can do chores. But don't be scared, it's not as "bad" as it sounds. My sons started out by having to pick up their toys before bedtime--which I help them with and, as they become more proficient, they pick up more and more on their own. (They also aren't given a large amount of toys at any one time, so the cleaning up doesn't overwhelm them). The other job that they do is taking the dirty laundry from their room and putting it in the laundry hamper every morning after breakfast. After meals, my older son (3 yrs)takes care of dishes, washes the table, and both boys clean up the floor. I don't expect perfection, but they're getting a valuable learning experience and they have a good attitude (most of the time) about chores because they've been doing them from as long as they can remember. They are two very active, not perfectly behaved boys, but they do pretty well in this area and they feel that they are a vital part of the family unit.
Dr. John Rosemond has written MANY, MANY books about child-rearing and devotes very large portions to chores and why they are such an important part of a child's upbringing (RESPECT, RESPONSIBLITY, & RESOURCEFULNESS). Read "A Family of Value" by Rosemond for more information.
Hope this doesn't sound "preachy." It's hard to convey everything in a quick message.