How close can a trademark be to a famous brand?

To bolster Mr. Weinrieb's answer, which is a good one, there actually was a case in the Trademark Trial and Appeal Board of the US Patent and Trademark Office in which Google successfully opposed the registration of the mark SOOGLE for “Arranging and coordinating travel arrangements for individuals and groups… arranging for travel… documents for persons traveling abroad; arranging travel tours… online trip and travel reservation services.” See In Google Inc v ACPS Global, Inc.
Trademark infringement is highly fact-specific so there could be many reasons why these marks are coexisting or perhaps they are not coexisting and you are not aware of any pending or pre-litigation activity between these companies. For instance:
* Is Forever 21 operating in Germany and therefore possessing trademark rights that conflict with Forever 18? Or perhaps Forever 18 existed in Germany before Forever 21? Trademark rights are territorial.
* Are there many clothing stores or clothing brands that use the term "forever" in their mark such that the number 18 or 21 distinguishes those marks?
* Could there be some relationship or affiliation between Forever 21 and the user of the Forever 18 mark such as that Forever 18 has a license from Forever 21?
* Maybe Forever 21 has initiated contact with Forever 18 to demand that Forever 18 stop using their mark?

If you have a situation like this with your business, you should consult a trademark attorney in your jurisdiction to review your specific facts.