There's really two separate questions here:
What age is the content "appropriate" for?
At what age will the reader have context to understand the content?
Like, "Spending a year dead for tax reasons," is good enough content for someone who is old enough to handle death as content, but the phrase doesn't mean as much if you don't pay taxes.
One way to look at the HHGttG is as a piece of absurdist literature, but that requires you to have enough understanding of the normal world to get what is, and isn't, absurd.