here a link to the youtube T&C's www.youtube.com/static?gl=GB&template=terms
Here's the google age T&C's (min 13)support.google.com/accounts/answer/1350409?hl=en
In practice unless somebody reports Google/Youtube ignores the T&C because it's user generated content which drives traffic.
The issue here is that the OP is (IMHO) talking about content creation, not just watching Youtube. The practical reality is that any kid is capable of creating video's given the right equipment for them. It'show it's then managed and distributed.
We manage what our kids post up to youtube, they can only do it with our permission and concent. They may think that the channel is theirs, it's actually a separate account that me and the OH "manage" AKA behind the scenes control-freaky parents who watch over what the kids are doing.
Our 10yo daughter knows full well how to set up a google account and could if she really wanted to, she also knows how to record stuff on a phone camera, or on a laptop or even with her own digital camera. And she's at an age where she can read and understand the prompts. If you throw them a camera and then throw them over a wall and let them get on with it, you could be in for a world of misery.
The upside is that either way they could become a youtube star and be minted before university, or they could just have some fun and learn something along the way. Or they could discover it could be something like hard work and give up.
My personal feeling is that I like to keep an eye on it so I don't miss out on something that they are up to.
Also this could be an opportunity for them to learn internet safety, and what goes around can come back to haunt you. By the age of 7 our eldest knew what to look out for when it came to internet creepy people, she has quite a talent for spotting grown-up men pretending to be little kids, boys pretending to be girls and the mean and rude - and this is how she's described it to us, when she calls us over to check something out.