When you create the account and set the age (right on the first page where you add the name) - That's the age I've found which dictates everything.
For me child profiles display content that has age rating lower than the age you select for the child profile on the first page.
I'm guessing that if you change the age to something like 55 it would show you everything. However, that's risky because you'd have your kid run into a load of very messed up titles and covers that are present in most ebook stores i.e. really sketchy stuff I'd rather not mention. I'm not even talking about normal erotica.
I think setting age as 18 is a reasonable compromise.