A study published last year in the American Journal of Psychology collected “data about the emergence of adult like swearing in children.” The authors, Timothy Jay and Kristin Jay, recorded observations of children ages 1 through 12 and adults using taboo utterances, which “were described as offensive words and phrases (e.g. fuck), insults or name calling (e.g. douchebag), and clinical terms (e.g. penis), as well as abusive expressions (e.g. I hate you).”
The study found that, overall, boys had a slightly larger repertoire of bad words than girls (95 compared to 80). But that repertoire varied by age. By age 3 or 4, girls were using 40 taboo words while boys were using 34; but among 7- and 8-year-olds, boys were using 45, and the number of bad words girls were using slipped down to 25.