How close can a trademark be to a famous brand?

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.