Katy and Rihanna in the sense that all of their music doesn't evolve much with each era. Katy is always giving us guitary-bubblegum pop and Rih is always giving us R&B with pop elements feat. some rapper. (WFL was an exception)
You could include Ke$ha in this list, but since shes only had 1 album, with a companion EP, it's impossible to determine her growth until we actually hear "Warrior"
None of their music is immature. And i'm failing to see how taylor swift's music is immature, seeing as her themes about love and heartbreak are still sung by artists way older than her (and their praised for it).