The key point about the Internet Content Rating Association (ICRA) is that they themselves do not rate any website - it is down to the website operator to do that. (See click here for more detail).
Therefore, the scheme relies on 'self-policing'. This is bound to fail with most (but not all) 'adult' sites being solely interested in attracting as many visitors as possible, in order to raise revenue. An advert on an adult site cannot know whether it has been viewed by a 14 or 44 year old, and will pay up regardless - this is all that matters to most site owners.
There have been many discussions on this forum about children accessing undesirable content on the Web - use the search facility to search for terms such as child, children, kids etc.
Note that if you go to Google's home page and use the Preferences link, there is an option to filter search results to avoid undesirable content being returned.
See also click here