Right, how about this is 100% Google's fault for not figuring out how to avoid offering contracts to children. Seems more reasonable than anyone else getting any amount of blame or punishment whatsoever, including and especially the minor involved.
On every terms of service document there is a clause that says "we can terminate our service to you unilaterally with no warning for any reason including no reason". It's basically the catch all exception handler of the legal world these corporations live in.
If you need to be at least 18 to use a particular service, and they're going to verify at some point, that point should be at the very start.