Hard to believe Kate Bosworth is 30. But she is, and the Cohasset-bred star of such films as “Blue Crush,” “Beyond the Sea,” and “21” celebrated in style. Bosworth was surprised by her fiance, Michael Polish, with an opulent Roaring Twenties-themed birthday party in LA. According to People.com, Bosworth’s friends (including Dita Von Teese and Robin Antin) and family (including her parents), toasted the actress. Guests dressed in period attire, including flapper dresses and long necklaces, and a big band and an aerialist performed for the crowd.