How close can a trademark be to a famous brand?

It's hard to definitively state what is and what is not considered to be an infringement of a mark - on its face, it would seem that "forever 18" and "forever 21" would be confusingly similar if in fact they were marks used in the same field of goods or services. The rule of thumb usually is that the more famous the mark, the more well-known the mark, the more protection it would receive. For example, while everyone knows Google, and while Google originated with search services, Google could effectively today enter into any field and people would immediately associate the particular goods or services with the original Google corporation. In a similar manner, if someone else started using the Google brand/mark for even diverse goods or services, the real Google would probably prevail in an infringement suit because the mark /brand Google is so strong, therefore it is accorded extremely wide protection.

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.

There is a possibility of similarity. And as Steven said, both of these are leading to confusion, Because both of these sounds like a series of marks.

In simple comparison
- Forever = Forever
- 18 & 21 are numbers
- 21 just includes 18 and demonstrates superiority by addition of 3, which actually trying to match the baseline concept of forever 18.
- To add up, in specific countries 18 is a major age for the Girl, and 21 is boys major age, . If so, this indirect discrimination may be one of a concern to argue for the similarity.
Example : "adidas young" and "adidas adult" cannot be said non-similar.