Translation:The majority of adults in the United States know how to speak English.
Fun fact: The english word "adult" comes from the latin perfect passive participle "adultus" of the verb "adolesco".
Latin verb "adolesco" means "I grow up". The passive perfect participle "adultus" means... "grown up".
Why esperantists would prefer to say "plenkreskulo" to "kreskitulo"? I don't know. But you can see the words in esperanto and in english have about the same etymological meaning.
An other fun fact: The active present participle of the latin verb "adolesco" is... "adolescens". An adolescent is someone who is "growing up". According to Google, no one seems to have ever used the word "kreskantulo", though....
In another lesson, someone pointed out it's just an agglutination of Plena(full)+Kreski(to grow)+ul(person)+o(noun); an adult is a fully grown person noun. I'd presume a kreskitulo would be interpreted as an a compound attempt for adolescent/teenager when you can't remember the word, a "growing person" who isn't being described as an infano.