Yes, they can. Adults learn sign language quickly because the process of learning a new language is largely the same for adults and children. The biggest difference is that adults tend to have more patience and are better able to focus on tasks for longer periods of time.
The best way to learn sign language is by using immersion methods. This means surrounding yourself with as much sign language as possible and using it in all aspects of your life. You can also find online resources and videos that will help you learn faster. If you’re willing to put in the time and effort, you can definitely become proficient in sign language!