Good point. By the time the kid turns 21, C will be over 50 years old.
C is eternal, or has been up until this point and certainly the low-level language if you want to learn one. C11 isn't the C from 40 years ago. It's just not going to be relevant enough for most tasks, going forward. It also won't be going away, which is why it's still the low level language to learn if you so desire.
C was developed in an era where extracting every bit of performance out of your hardware was mandatory. Today, that kind of performance tweaking doesn't matter as long as you have browser that runs on it. To put that into perspective, I'm typing this in a browser, in a virtual machine right now, on a ~5 year old CPU.
As far as we can tell, Javascript is the new C. Yeah most of us don't like it, but its time to bite the bullet and give up resistance IMO. Python is good to learn on, but Javascript is a must-have unless you want to render yourself obsolete. I suck at JS myself, but it's a true statement.