I'm 20 years old and I am definitely not an adult yet...in fact I see a lot of myself in Lauren, I feel like she's at a vulnerable point in life where she's working on figuring out exactly who she is.
Maybe she doesn't have a clear sense of identity and so she feels like she has nothing solid to rely on in the house, so things affect her more.
20 is still very young, and while it doesn't mean she should use that as an excuse to be childish (as Becky does - 'I'm only 19' -
), I think you should cut her some slack.