Secundum ad US Census hendrerit aestimationes, illic erant 41.910.114 iuvenes aetate 10-19 In Civitatibus Foederatis Americae, a summa XIII% of US populatio, in MMXVII.
Short answer: 42K.
The America population exceeds 1 Billion. As [ Una iuvenis femina ] ceases to be an unmentionable in Neo • Contemporary Latin • Latin • America • then 1 billion of those aged 0-127 years may wish to be identified as un un/a iuvenis. Those exceeding age 127 years are arbitrarily designated as seniores and a fetum is designated as a iuvenis wannabe.
Why do all these sentences about young men feel like Duolingo is looking for Hot Dates?
Ha, ha... I put "youth" just to expand their English database. It's more inclusive of young women etc.
Does this course really need all these America references? It would be much more interesting (and useful!) if locations like Greece, Gaul and Carthage were used to practice declensions.