When I was 15, my brain was definitely pubescent and far from fully developed.

But I was able to make some money by fixing computers or translating stuff from English to Czech anyway. There was no exploitation in those labor relations just because I was young.

I am not manually skilled, but I can definitely see someone at 15 making a nice chair or a table instead.

I don't think that 15 y.o.s should be treated as fully adult, some limitations on their work are perfectly OK (no ardous work, no work underground etc.). But barring them from working altogether will probably slow their development down. Not everything can be learnt from books or models, some real-world practice, including the most basic elements of interaction with customers/employers, is necessary.

Looking at the level of independence I and my peers handled at this age vs what is the norm now we might have overdone it.

Isn’t that just the average though?

I’ve met plenty of wickedly level-headed 15 year olds and a whole lot of irresponsible 30 year olds.

The variation is such to an extreme level too.

Yes, but the same is true (a bit less so) of an 18 year old and most places allow 18 year olds to work, drive, vote, join the military, enter into binding contracts as adults etc.

While teenagers are not fully adult in some ways, they are also very different from a 12 year old.

I mean, to some extend, but I don’t necessarily think that working is bad for a 13 year old. If they can work in a supermarket to earn some side income they can work anywhere (under limited guidance).