8 and 5
It wasn’t until the middle school groups (age 12-14) that you really saw kids with what looked like genuine concern for others, and we ran staffing levels more like 6:1.
I grew up in the 80s and 90s and remember spending a lot of time unsupervised going about wherever and doing all sorts of wild things well before I was 7 years old. Looking back now as a parent it is absolutely terrifying to me what I was "allowed" to do. Then again when I think about the Norma in countries like Japan (where it is not uncommon to see kindergarten kids walking home by themselves and crossing streets and intersections), I am reminded that kids can develop independently to a great tune than we give them credit for. I don't know about Japanese kids materials levels but I've noticed that they are typically given more freedoms to act independently and spend a lot more time on outdoor and social activities. If I had to guess I'd say they are a lot less materialistic than their Western counterparts.
As for my own child, I try to let him be independent even at 1 year. For example, we have a TV stand that any parent would be terrified if their kid climbed on it. I was terrified at first but then with some observation and secret supervision I let him climb it. To my surprise he quickly learned how to turn around and get down safely. There was only a single time when he fell from it sideways unhurt, but now he goes up and down like a pro without any supervision. I can leave the room knowing he will always get down fine. To what effect independence will have an impact on materialism remains to be seen but I suspect one of the commenters is right on the money with mirroring behavior and values.
My experience with Japanese children is limited, but when I worked in informal education we had a number of groups of high school students from Japan, and when I was in college there was a large group of exchange students from Japan. Some of my American friends went on to become full-time teachers in Japan… and some of the Japanese people I knew went on to become full-time teachers in America. My impression is that if you think that Japanese children are “given more freedoms to act independently” then this is probably a result of your cultural bias around what it means to be independent. The phrase that usually gets thrown around is “出る釘は打たれる”, which means “the nail that sticks out is hammered down.” There is generally a lot of structure in the lives of Japanese children. As for materialism—your mileage may vary. The Japanese people I’ve known haven’t been exceptionally different from Americans when it comes to broad aspects like materialism. Japan definitely has its share of brand-conscious consumers.
And this is a song and dance we’ve heard before. “If I only raise my child using principle X, then they’ll grow up to be great people.” There is no secret technique to raising better children, there’s just a million little things and a lifetime of effort.
It's mentioned by Piaget that kids are inherently ego-centric at that level, and likely won't share. It think it's a generally relative thing for them to be very self-centered at that age.