I'm over 15 years old. But I am very sorry that I did not start to be interested cryptocurrency and investing early. I think that 15 - 16 children, it is just the right age for it
Many peoples regret because the same reason.
But, fortunately, i've known bitcoin since i was 13, and i start investing from now.
I think children can also use bitcoin. Just teach them. And most of them will be interested to bitcoin. They can invest as small as they can in order to learn how to earn with bitcoin.