It’s hard to tell by looking at her, but Susie is much older than Ramona. But Ramona is the mature one, even able to handle working with Susie with grace! Something about Susie’s immortality keeps her mentally a teenager as well, not just physically.
While a lot of Susie’s negative traits tie in to her being stuck as a teenager, not all teens are the same, and Susie doesn’t have to stay the same either. She’s becoming a better person (though maybe a little too late for Emma’s liking).