Although I notice that several web browsers in iTunes have 17+ ratings too, so at least they're consistent, maybe. According to https://itunes.apple.com/us/app/mercury-browser-fast-web-bro...
You must be at least 17 years old to download this app.
Frequent/Intense Sexual Content or Nudity
Frequent/Intense Profanity or Crude Humor
Frequent/Intense Mature/Suggestive Themes
Frequent/Intense Cartoon or Fantasy Violence
Frequent/Intense Horror/Fear Themes
Frequent/Intense Realistic Violence
Frequent/Intense Alcohol, Tobacco, or Drug Use or References
As for children having access to the built-in web browser, certainly not if their parents don't think full web access is appropriate for them.