I have an uber-helpful 7 yr old.
I mean, if it's there to be done she'll do it--whether she's physically capable or not. And it totally ruins her day if you say no. I mean, seriously. Her emotions are all or nothing. For the record, I have yet to SEE the "nothing" stage. She's seriously awesome, if a little intense. :)
Conversely, my 9 yr old daughter behaves as though helping out is a teeth pulling exercise. She'd rather be doing ANYTHING else, which is why--in large part--I want to be sure she's part of this new project I'm working on. I want her to realize the joys of having a charitable heart, so that she can grow up to be a compassionate person.
I know a lot of these stages are age related, but my older daughter has always been a touch more self-absorbed than the other kids. So I realize teaching her charity will require more effort on my part than reinforcing the behavior of her sister. It's just further proof that children, even when raised in the same environment, are individuals who will process the same bits of information differently.
Kids are cool. And they are worth the effort. I love both my girls, but there's no way I'm going to say they are the same. :)