bright young thing
An unconventional young person from the 1920s and 1930s who lived to go to fashionable parties, drink and shock their elders. As with their modern counterpart the hipster, this madcap lifestyle often depended on a regular allowance from wealthy parents.
I remember my great grandma Hazel as a grouchy old lady who knit all day, but my mother said before she got married she had been a bright young thing who drank bootleg gin and swung on chandeliers.