I
LOOOOOOOOOOOOOOOVE Calvin and Hobbes!
IMO, there is no age that is too early.
Yeah, in the latter years of Calvin and Hobbes, Watterson got more existential which might fly over younger readers' heads, but Calvin is a timeless character and the earlier strips especially are perfect for younger readers.
I was about 8 when I first started reading Calvin and Hobbes. Those strips were as much a defining moment in my life as dinosaurs, Ninja Turtles, and Star Wars. I used to pore over those strips for hours, examining every ink line, word, and (in the case of the 10th anniversary book), comment Watterson had and would often get in trouble for being caught under my covers with a book and flashlight at night.
What was great was that as I got older, I began to understand more and more of the strip, and by way of that, it was like reading them for the first time all over again, which was a wonderful thing.
I think waiting till 14 would be stealing many years of Calvin and Hobbes enjoyment from a kid. There isn't anything that would hurt a younger child, and if anything, its a great way for the younger crowd to learn to love art, reading, and comics in general.
So I say, let em read the strips as early as possible.