Garland, the star of "The Wizard Of Oz," came in second and Johansson, who is also 21, came in third, Life Style Extra reported Tuesday.
A spokeswoman for the poll conducted by Yahoo!.com, said, "A starlet is an actress who people instantly think of as being young, vivacious, stylish, popular and, most of all, talented."
"There are a fair few starlets jostling for space at the moment, but Keira is the one who embodies all of these attributes. Scarlett may be the talk of Tinseltown but it is Keira's class that leads the pack," she added.
Notable deaths of 2014 [PHOTOS]