[kqr]

Also I see a lot of playgrounds that are designed according to the rule that "if you can reach the thing on your own, you are old enough to handle the risks of the thing".

It's quite brilliant. Very small children up to almost teenagers can play on the same structure and challenge themselves in appropriate ways for their skill, because of the initial skill threshold in reaching the more risky parts.

I'm the first to admit that even as an adult I have had fun on those, by climbing the exteriors and leaping across long gaps. I hope it benefits my children that I can play with them that way.