I am not legally trained at all, so I welcome correction, but here is how I see it.
Technically speaking it is against civil law, not criminal law. The reason for this is that if you are not of legal age for your consent to the Terms of Service to be valid, then technically you are using Google without having agreed to those terms.
If they don’t put in the stuff about being of legal age to agree to the terms, they could have a scenario like this:
You are a 15-year-old web designer. You use Google’s logo and website design to create a spoof site that looks like the Google site. You reverse-engineer parts of the Google search engine, and then use your site to gather information about people and sell that information to unscrupulous marketers.
Google finds out about it and tells you to shut it down. You say you won’t. They say that you have violated the terms of service for using any Google resources, and they will sue you if you don’t stop. You say you have no contract with them that binds you to any terms of service, and therefore you can do whatever you like with their resources. They say you have to have agreed to the ToS in order to use their service, and you say that you did, but since you are a minor you did not have the authority to enter yourself into a contract with them and the “contract” is invalid.
Without the various terms in the ToS, their avenues for redress are limited. They can still go after you for some sort of copyright violation and so forth, and of course they can tell your web hosting people to take it down and they almost certainly would. But it’s more complicated, time-consuming, and uncertain.
With the terms in there, they can come back and say, “You have no right whatsoever to use any Google services or graphics or anything, see, it says so right here. Therefore you have to shut it down anyway.”
So, basically, this is a CYA against people who want to misuse Google property. As long as you actually adhere to the ToS, nobody is going to care about your age.
Anyway, that is what I think it is about. I welcome more exact and/or correct information from anyone with legal training.
–Jen