Little girls start believing gender stereotypes by age 6. 

A 2017 study found the stereotype of brilliance being more associated with men influences children as young as 6, an age when girls begin to ignore activities that are said to be for ‘smart people’. 

At age 5, the kids in the study saw both genders as equals, but by age 6, the girls thought of the boys as more intelligent and avoided playing games intended for ‘really, really smart’ kids. 

(Source, Source 2)